The company definitely was not composed of contrabands, though I bet a lot of them were later in USCT units.

I know that the 1st Battery of Arkansas Artillery A.D. was formed from Contrabands at Pine Bluff. And if I am not mistaken several companies of the 5th and 6th Arkansas Infantry A.D. were also formed at PB.

The reason I speculated that maybe Capt Murphy Company may have been the early Genesis of these organizations was the fact that Monroe's Cavalry at the Battle of Pine Bluff captured and burned a large quanity of materials in the contriband camp NW of the Courthouse. The impression is given in Confederate reports that this materials were military issue types of blankets and such. I know that the Union army did not just give contribands blankets, tents and such, without some sort of obligation on the part of the Contribands.

I know that Capt Talbot was in charge of the contribands that were organized as work details that built the Cotton Bale defences and carried water from the Arkansas River to put out the fires.

Col Clayton's comment that Capt Murphy company "fought like veterans" kind of indicates that maybe they weren't expected to fight like that. As you said they were in fact fighting for their lives because if the Union defences had failed they would have been shot as Traitors. That traitor status would have been applied to both southern whites as well as blacks who were actively fighting under arms in Union service against Maradukes men.