Kopfjaeger
First off Blackhorn is a progressive burning powder. This means that the higher the pressure the faster it burns and produces more pressure.
Second to answer your questions. It is quite possible to get these velocities with Blackhorn by loading it extremely tight. Since the normal pressure for 120 gr of Blackhorn is 15 % below the normal pressure of 777 and it gets the velocity it achieves by burns longer at peak pressure it is probable that your load does not exceed safe limits this can not be told for sure with out pressure testing.
Because a load that causes bent ramrods is not convenient for normal use I suggest you try a slightly thinner sabot or polish your barrel with 100 full length strokes of JB Bore Paste, this should relieve the loading problem as polishing tends to make things slide easier.