Charly,
The most widely spread Tatia is apparently T. intermedia, a species which is described from the area you mention, whereas T. aulopygia isn't. Similar patterned species are T. galaxias and T. neivai. This is based on a description by Mees (1974) and can as such also be found in Sands' Volume 3.
I'm almost happy that even these authorities were puzzled. Mees even suggested to create 3 subspecies besides T. aulopygia aulopygia, namely T.a.intermedia, T.a.neivai and T.a.galaxias.
He later on withdrew that intention and returned to the 4 species mentioned above.
The problem is that all four are similarly patterned (dark body with off-white markings), but - as far as I know - all species except for T. aulopygia keep their pattern. I've had T. aulopygia, T. intermedia and T. neivai for quite some time, and only T. aulopygia became jet black. This is also the largest of the four (12 cm) and the male had an elongated upper tail lobe, which was twice as long as the lower one.
As your specimen is black, I'm sure it's not T. neivai or galaxias. When we discard T. aulopygia because of the distribution range, I could only think of a yet undescribed species and/or a variant of T.intermedia, which should then be called T. cf intermedia.
However, if they were offered to me as T. aulopygia I would say "well determined" and buy them on the spot
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