If there is still 15K psi pressure below the pipe rams the last thing you want to do is attempt to open the rams. The button head on the ram shaft could crack or worse, fail completely. It's common occurence for the test port in the test stump to plug off with cuttings or debris that accumulated in the ram cavities and annular cavities. If possible, it's very good practice to open all ram doors and clean/inspect rams before doing a pressure test, such as when subsea BOP stacks are on surface. For surface BOP stacks jetting the BOP ram cavities prior to testing would clean out debris.
You can try several things:
1) try to pump through the test line at very slow rate. It's sometimes possible to dislodge the blockage up into the BOP. Going 10-15% above rated working pressure of the BOP will not affect the equipment.
2) Open a failsafe valve, or wing valve, to relieve the pressure, keeping the area clear of the discharge. This will be a sudden, but low volume, release of pressure and will likely require repair of the valve seat seals. Use the outer valve to release the pressure, keeping the inner valve intact to hold well bore fluids after the pressure is bled off. If any piping is connected to the valve outlets it should be removed prior to releasing pressure. Using a valve to release pressure will cause less damage to BOP equipment than trying to open the rams.
3) To prevent blockages of the test port install a cover similar as described in Vetco bulletin H051491, attached.
4) In future bleed off the pressure very slowly. Sudden bleed down will draw debris into the test port.
Hope this helps.