Hey! :) I've been curious about a few Barbies I had from thrift stores as a kid, 'cause neither had clothes, so they wore whatever I had, so I never knew which ones I had. One blonde one had a giant pink piece of plastic sticking out of her back, one was a black haired girl with a purple highlight so I think? she would have been a skipper, but she was the size as the other ones, so I never was sure, and another one had a pink necklace? tattoo? with a heart in the middle of it, blonde. Sorry if this isn't very descriptive, I barely remember them.
This is a bit of a tricky one to answer definitively without being able to see the dolls - there's a lot of features they might have that can help identify them, like what era they came out, face sculpt, bodies, and so on.
That description of the black haired doll with purple streaks made my mind go to Fashionista 231.
However, I highly doubt this is it... because Fashionista 231 was from last year, so not consistent with your description of this being a doll you had from a thrift store as a kid.
So in all likelihood, it was a Skipper, and specifically, a Skipper from sometime post-2009 (as this is when she started rocking what certain friends of mine persist in calling Eridan swag). As you point out, the dolls were the same size though, and Skipper is usually smaller than Barbie.
There's a couple of possibilities from here. Skipper is usually smaller than Barbie, but is approximately the same size as Barbies with the petite body type, so there's a chance that one or more other Barbies you had were a petite and that's why they were so similar in size.
But I'm also mindful of the fact that you said your doll had black hair, and most Skippers have brown hair with a purple streak. There are dolls in the Skipper line with black hair, but with a blue streak.
With thrift store dolls, there's always another possibility though: they may have been customized - head-swapped, rerooted, etc. So if we really can't track it down, that could be one reason why.
That's not even getting started on the pink heart necklace or the pink plastic thing -- there's a lot of possibilities and a lot of digging to be done.
I know that was a lot of words to say "I don't know", but I hope that's an insight into my thought processes when people ask for help identifying dolls.
