I found a way around this. it is, most definitely a faff and a workaround, and is NOT a "how it should be", but it works nonetheless, so if anyone in the future is reading this - this is how you do it:
Make a new group for all the people you want to apply this permission to (I just copied my basic "members" group so they have the same permissions).
Give this group the "can edit own content" permission, click "customize", choose ONLY "Blogs", and tick the "unlimited" box under " Edit time restriction" (See 'screenshot_1').
Choose the people you want to have this permission. for us it was pretty straight-forward, I just used the "Download member list" and filtered by "Member has a blog". To do this, in the ACP go to Members->Members->Download member list (See screenshot_2) and the scroll down to "Member has a blog"->choose "yes" (See 'screenshot_3').
Put the people you chose into that group (you can import the list you made in the previous step to the new group).
Bonus Step: You can, additionally, grant this group the "can lock own content" and choose "Topics". (See 'screenshot_4') That way you can encourage your bloggers to open a thread for when they have a new blog post, link that blog post and have people who want to comment on it, be "forced" to comment on the blogpost itself and not in the thread. this allow you to KIND OF replicate what can be automatically triggered with articles, where when an article is published a thread automatically opens and the comments to both are shared between them.
This should, at least minus the member list download part and to the best of my understanding, work on any "type" of content you want to allow your members to be able to edit without any, or with different time limits.