The three engines that were designated for Blue Comet service were part of the CNJ's G-3s/P-47 class 4-6-2's built by BLW in 1928. There were 5 engines in this class numbered 831 thru 835. Only the 831,832 and 833 were painted in the Blue comet colors.
The specs on the engines were:
Drivers -79"
Weight on drivers- 326,000 lbs
Cylinders= (2) 26' x28"
Boiler pressure -230 psig
Tractive effort -46,841 lbs
Tender capacity - 10,000 gallons
All had 5 engines Elesco feedwater heaters, stokers, and mechanical lubricators.
The 834 was painted a dark green for a period for service on the Bullet.
The 835 was always black.
It was also common to find other locomotives at the head end of the Blue Comet as engine availablity dictated. This included any of the other Pacifics, Ten wheeler L7sa/T38 or L8/T40 Camelbacks and even the P-6s/A28 Atlantics, including the 592 which had honors on a couple occasions.
Here's a nice pic of the 831 at Jersey City ready for duty: