(Not a Teacher)
This is not an expert literary opinion, but I'd say blackbirds like ravens and crows have two prevailing associations. One with death and decay, arising from their omnivorous diet. They likely picked up this reputation from hanging around (no pun intended) the local gallows waiting for a free meal. The other is with intelligence or wiliness, as they have been observed using tools and taking clever approaches to obtain food.
That said, I don't think any connotation is being used here. It's simply comparing her hair to the jet black plumage of a raven.
Student or Learner