This may be odd

[aquaculture]

I have lately seen people tade in their plecos due to them being to big at my lfs, and have lately seen that every once and a while theres a pleco that comes in that is as black as night and greater than 12 inches in length and has similar pattern to the other traded in commons, what pleco could it be.

the pattern is made up of different shades of black.

a very sharp looking pleco.

 

[Warborg]

My first thought was Acanthicus hystrix when you said a 12inch black one

http://www.planetcatfish.com/catelog/species.php?species_id=217

Check out this other thread on different commons

http://www.monsterfishkeepers.com/forums/showthread.php?t=271879

 

[aquaculture]

no sadly it wasn't an acanthicus, but it did have spiny rows on its back instead of just the armored plates, it could have been 9-12 inches , eyeballed the measurement, was in a 90 gallon it was longer than half the width of the tank though.
thanks for the help warbough and it also isn't any of the plecos in the thread you mentioned and planetcatfish ssems to be having a server error.