Blackrock Federal Penitentiary is a federal prison presumably located on Great Bear Island. The prison itself is not featured in The Long Dark; however, a Prison Transport Bus from Blackrock can be found near Milton in Episode 1.
Sometime during or after the First Flare, when the aurora first appeared in the night sky, a Blackrock prison bus carrying a dozen inmates crashed at the mouth of the collapsed mountain tunnel outside the town of Milton. At least one of the inmates attempted to escape, and his corpse can be found a short distance away from the bus. The status of the other inmates and the corrections officers accompanying them is unknown. However, in Story mode, Grey Mother cryptically mentions a group of people chasing a stranger who passed through town shortly before Will Mackenzie arrived, possibly referring to Dr. Astrid Greenwood and the inmates, respectively.
After surviving a plane crash, Astrid seems to have passed through Milton and somehow managed to get through the collapsed tunnel using the Blackrock bus, presumably leaving a message behind on the bus's floor--a single word written in blood: PERSEVERANCE.