An issue with the Blackhawk velocity not getting high enough can be caused by a mechanical, or air input issue. Always point the marker in a safe direction and always wear eye protection then check for these issues.
Is the velocity screw adjusted too far in or missing? Turn the velocity screw out to increase velocity. If the screw is missing it will allow air to escape and reduce velocity.
Is there enough air in your tank to operate the marker? If your HPA tank is below 1500 psi it may not be outputting enough pressure to operate the marker. CO2 tanks will stop operating at around %20 capacity.
Is the gun dirty or unlubricated? Any debris like dirt or broken paint will slow the bolt and hammer down and prevent the valve from opening long enough to operate the marker. Clean the inside of the marker if necessary and add Valken Tango gun oil to the hammer and valve. Watch these videos for more info:
Disassembly and assembly
Is the tank's output pressure high enough to operate the marker? The Blackhawk requires 850 psi output to operate. Low output tanks and malfunctioning tanks will output too low.
Is the Valve tube or main spring worn out? If the Blackhawk is a few years old or has been used many times the main spring may be worn out or the valve tube may need to be replaced.