You are more or less correct on the colouration of the slings.
Here is a link to the latest breeding of T.stirmi by Hendrik - there are nice photos of slings and the adult female
www.tarantulas.co.za/forum/breeding-repo...rt-theraphosa-stirmi
Also, when owning one, you can clearly see the sheer size difference of these T's at 2nd instar, compared to other tarantulas. And they are really almost pitch black/dark grey, so anyone trying to sell you a "brown/dark brown" sling as a Theraposa is trying to rip you off!
When reaching adulthood, the T. blondi & T.stirmi looks quite similiar (they both look stumpy), whilst the T.apophysis is much darker (and looks leaner!) The T.blondi has hair on the patella (I hope I used the correct term here!) i.e. the knees on the front legs, whilst the T.stirmi sports no hair on the patella.
But they are really magnificent creatures, and if you can get your hands on one, don't think twice about it!!