If you previously had a graphics card in the computer make sure you completely cleaned the drivers out then reinstalled. Also make sure the temps at idle and under load are under maybe 50C/90C. I only mention idle because I'm assuming it blackscreens under load. If all is fine my guess would be you don't have enough power to power the GPU.
I come up with a recommended 260 watts of power for your system. 240 probably would be ok new, but that power supply is pushing 9 years old, so now probably significantly less.
I'm guessing everything works fine at idle or when web browsing, but then on anything GPU intensive that's when the problem starts?
I actually had to research this one. The motherboard has a weird, even for 6-pin connector, that would require not only a 24-pin to 6-pin adapter, but rewiring of it. The form factor is a very strange one, form factor CFX, so you would have to most likely house a new power supply outside the case.
https://h30434.www3.hp.com/t5/Deskt...te-8300-SFF-psu-graphics-upgrade/td-p/5743592
Bottom line, this is one of the most proprietary and oddball motherboard and case combos ever made.
Me personally, I'd either sell it and buy a newer refurb with a full ATX case and 24 pin power supply connection on the motherboard OR
Source a used non-OEM 1155 motherboard, new full ATX case, new power supply, and when windows unregisters itself because of the new motherboard, call Microsoft and tell them the motherboard broke and give them the model/serial off your case and they'll give you the code to re-authenticate it.