Blacksuits are the guards of the prison named so for the black suits that they always wear. They have a notable stature due to the many surgical procedures taken out on them by The Warden . Outside of the prison, blacksuits are also officially recognized as a PMC (private military company).
All the blacksuits look so incredibly similar that they are often indistinguishable, save for very specific skin conditions (moles, birthmarks, tattoos, etc.). They have silver eyes, and grayed skin with veins that appear black and are all incredibly tall and muscular.
All blacksuits were 're-birthed' from the prison through a series of surgical and psychological operations. The perverted process uses abducted prisoners in order to provide a body and is carried out by the Wheezers in the Infirmary.
Through the eyes of Alex Sawyer, the long and bloody surgery takes place. Starting with his legs, muscle tissue was carelessly implanted into his body. During the procedure Alex felt excruciating pain. His body was scrawny and frail at the time of capture but was now supersized. To compensate for the loss of blood and give him the final step of power to become a blacksuit, the gold-speckled 'nectar' was injected into his bloodstream.
During his recovery from the surgery, he was placed in a mostly empty room with only a set of screens and a chair with securing belts. It was here that the psychological process was carried out on Alex. he was kept in the room for a series of days, possibly even weeks, while violent imagery was played onto the screens. The Warden consistently came to check in on his cooperation so far. Alex held out strong against the psychological torture but was eventually changed mentally into a blacksuit, willing to do the Warden's bidding.
Blacksuits have a utilitarian purpose of defending the Furnace Penitentiary from its dangerous inmates inside. This job is rather unchallenging for them considering their size and their armaments, including Beserkers, Dogs, high-powered shotguns, and their surgically enhanced bodies.
On a more disturbing note, Cross genuinely believes that the blacksuits are to purge the world of all weakness in a final great world and create a Fatherland of complete power and fascist control with his new master race.
The Blacksuits are known to have a very violent personality and are usually unafraid of the prisoners. They also are very defensive of their 'brothers', as seen when in the large scale riot and escape staged by Alex Sawyer they became enraged after their comrades were killed. This is largely due to being completely brainwashed to the idea of becoming the apex predator the human race needs in order to survive. 
The blacksuits in Furnace's tower have the same personalities and ideological ways of thinking as the ones in Furnace, but think of themselves as being leagues above the blacksuits in the prison, referring to those guards as "mere prison rats".
Furnace claims that only Alex and Cross are the only two people who have been able to handle the surgery and the power of the nectar yet still a solid grounding of their former lives and memories. While hard to argue the extent of their remembrance due to their limited screen time, it is interesting to note that the same could be said for Monty and Sam.