My feroli combi boiler pressure?

Hi, I have read through comments from previous questions but everything I have tried so far to reset the water pressure on my feroli combi boiler has not worked for me. I have located the valves to turn and followed the instructions given to others with the same question, however, I cannot turn the black valve below the gauge fully as it pours out water all over the place...is this normal? This is my second day of trying to fix this problem myself.