Firstly, I believe calling these something like C. cf venezuelanus "Northern Guarico State" instead of C. cf venezuelanus "black" will stop some of the confusion in the hobby between this rarely (if at all) kept fish and the various other "black" cory spp either natural or man made.
C. venezuelanus is no doubt a valid sp. It is clearly identifiable with a distinct limited range in two closed drainage systems (Rio Tuy system and Lake Valencia system) situated north of the serrania del interior. As I noted before I believe this makes it the only cory found on the north side of the Andes chain. No other cory spp exist in those two drainages. The habitat is cool, clear water that rarely rises above the low to mid 70s. Currents run fast in most of these streams and the substrate is typical of high gradient streams (rounded stones and gravel).
Perhaps most importantly for our discussion, no fish spp to my knowledge from the Tuy/Lake Valencia drainages is found in the Orinoco system on the other side of the serrania del interior. So if C. venezuelanus and C. sp "Northern Guarico" are the same sp this would represent the only known instance of a Tuy/Valencia fish sp existing in the Orinoco basin as well.
C. sp "Northern Guarico" is limited to the northern Orinoco feeder rivers with their headwaters in the serrania del interior. These are true tropical rivers rarely seeing temps below the mid 70s. This sp is most often found amongst leaf litter or mud substrates and can be found in the company of other llanos cory spp. It is smaller than C. venezuelanus. I do not have a series of measurements to show this across the two populations but it was something I always noted.