I have seen a variety of therapists. They all had different boundaries around in-between session contact.
My therapist that I have now is much more comfortable with in between session contact than I am. We settled on my emailing her things that come up during the week, anything I want, and she reads it whenever, and usually doesn't respond until we talk about it in session. I keep the emails to 1-2 per week. Occasionally she does respond but I never count on it, and we always talk it through face to face.
We text if I need crisis help, i.e. my life is in danger and I need her help, or urgent matters about scheduling.
My previous trauma therapist initiated and encouraged in-between session contact. She was an amazing therapist too.
Another therapist that I saw in the past also encouraged this (but she was terrible overall.) Both those therapeutic relationships ended badly and abruptly, and it was in part due to things that got too stirred up / misocnmunicated on text. It happened over 18 months into the relationship with the good therapist, and it was sudden and confusing and horrible. The texting was such a great tool, until it destroyed the relationship really fast one day.
That is why with my therapist now, we always talk through emails I write face to face where any misunderstandings can be worked out. I'm super scared of that happening again.
That's just what works for me. It gives the benefit of being able to have some connection to help me open up and yet also avoids problems I have struggled with in the past.
I had another therapist in the past that didn't have any in between session contact, but I could bring things in for her to read.
I know other therapists who exclude all in-between session contact, except in case of emergency. And there are some who do not do even emergency in between session contact.
There really is a spectrum. Every therapist has different boundaries that they keep, and in the US, there isn't completely cut and dry standards regarding the amount of in between session contact is a good idea or not. It's very contextual, and depends a lot on the particular therapist client relationship and the context and content of the communication --- and especially what therapeutic goals are being served by such contact.
In your situation three things raise a red flag for me:
1.) She feels it blurs the lines.
This is probably the biggest red flag. Whether it would blur the lines for other therapists and client sort of doesn't matter. For some it would, some it wouldn't. What's really important is that she feels it blurs the lines in this relationship. That's a good sign something Is a little off.
Therapeutic boundaries are not there just to make things hard, but because it keeps everyone safe and actually does add to the effectiveness of therapy over the long haul. If there is in between session contact, it should always be done in a manner where the boundaries or lines of the therapist/client relationship are intact. This is actually part of why some therapists do allow contact, because for them they can maintain the lines, and in some cases it even helps reach therapeutic goals and keep therapeutic boundaries. Other therapists, for a wide variety of reasons, don't do in between session contact because the lines do get blurred for them.
The best therapists know their limits and keep to them. They don't always know them in advance, but they figure it out as things go and stay within their limits as therapists and keep those therapist/client boundaries clear.
2.) She feels it blurs the lines and yet continues anyhow.
I do believe she is doing well at helping you trust and wants to do right by you. She's also human and this may just have gotten a little off track and it probably seems harmless. But the fact that she feels it blurs the lines but she continues suggests she is perhaps dealing with positive counter transference and possibly struggling to keep the lines clear for herself.
I think it's worthwhile to talk to her about why you both continue even when it seems like lines are blurred, and if that should change or if things should stay the same.
3.) The relationship feels more like a friendship to you as a result of this communication.
The best trauma therapy I have had is with two different trauma therapists who I came to trust, and sometimes view as coaches, sometimes as teachers, and sometimes they have felt like real jerks. No joke. They have not actually been jerks, and I knew that they were not at the time it felt like they were, but wow, it sometimes was so hard.
Good trauma therapists will sometimes be confrontationally kind and challenge us, and sometimes call us out on our stuff. I get a lot more annoyed with my therapist now than I ever do with friends... and it's also the most effective therapy I've ever done.
It's not entirely bad when a therapist feels like a friend, and it can actually help deepen trust. Much like you have experienced. Positive transference and feelings like that are useful and good. For some people, therapists can feel like parents or other kinds of transference can occur too. Its also good to feel the freedom to be mad or even be annoyed with therapist when doing the hard work of trauma therapy. For many, this is an important part of the process.
With the good therapist who imitated contact with me between sessions, it did help me trust her much more deeply, but it also made it harder to work through some issues - and I didn't even realize this at the time - until deep into the relationship and it kind of fell apart so quickly.
It's ok for the therapy-client relationship to feel like other relationships, even to feel like a friendship, as long as the role of therapist and client is very clear and understood by the client and the therapist maintains boundaries to keep their role a professional one at all times. The feelings can change, and be whatever they are, as long as the roles stay bright and clear. This actually allows for more feelings and things to come up that will help over the long haul.
Contact between sessions, however it happens, should have a clear therapeutic reason and be a part of a treatment plan, and not blur those lines of the client - therapist relationship. With the therapists that would initiate contact with me, it went well when it still had a clear therapeutic purpose. It eventually caused the end of the therapy when it stopped being that was and became causal, like I would communicate with a friend.
With my therapist now, she occasionally emails back, and we sometimes joke and banter, but not much of it. I'm in therapy not to do small talk or banter, but to heal. We do have communication outside of sessions, but we discuss it from time to time and check on how are relationship is doing often. We adjust as needed over time.
I'd highly advise checking in during face to face sessions on how this communication in between sessions is working for you and her. It might be that you both continue on just as you are, but have a good conversation to restablish and be clear that you both know this is a professional relationship.