Hi Onyx, we should keep an open mind to all hobby Brachypelma vagans or Hobby Red Rumps as I like to call them. They have been interbred with so many different Brachypelma spp. and none of our hobby spiders have got any DNA for a Brachypelma vagans. anymore.
Looking at '2007. The Journal of Arachnology 35:278–292' there is evidence that wild Brachypelma vagans. have DNA of some other Brachypelma spp. and in this experiment they found Brachypelma angustum DNA in some colonies of Brachypelma vagans.
What you have there is a hobby Brachypelma vagans and like cascade said it will darken up and also the older these spiders get the more they look like their wild cousins.

Onyx wrote: @Huzi I would have to disagree there I have a MM vagans and its carapace is black so not sure it is as simple as that. But it is interesting that its not as unusual as I thought if you had one as well

My male looked the same but upon final molt it turned jet black .

Let us know what yours turns out ,will be good to know