MoFed, you left off the first part of the quote: "Most of the negroes had arms,rifles, muskets, sabrs, bowie-knives, dirks, etc. They were supplied, in many instances, with knapsacks, haversacks, canteens, etc., and were manifestly an integral portion of the Southern Confederacy Army." Granted, the majority were teamsters and such, but the above describes soldiers, in whatever number.
Steiner's estimate of 64,000, according to your source, was twenty percent off. Given the same error in estimating the blacks he saw, that still accounts for about 2,400. His estimates appear far closer than Pinkerton and McClennan's, of 120,000 to 150,000, as you state; that's a 240 to 300% over estimation of what McClellan's army actually faced.
At this stage, it is likely the numbers will ever be accurately stated.
Another question: I have heard it said there were about 180,000black union soldiers. That's not a huge number, given the possible adult black males who could have seved. Is that all Lin coln could get, or all he wanted? No ax to grind, I'm just curious. Stan