Fluid retention causes expansion of the tissues to the shaft, creating slack skins. For appearance wise, tight skin during erection is more appealing to females than you think. Fluid retention is painful. It's practically skin stretching. It looks like weighed water balloons in your pants. Too much fluid in your tissues create too much concentration of blood clotting factor. Not good to have it there for too long. Skin discloration also occurs as the fluid contains pigmentation as the collected fluid is from blood.
To get rid of blackish penis? What is blackish penis? As in discoloration due to tissue trauma under pressurization? Discoloration can be common due to too long of blood trapping without cycling out. Blood loses oxygen, and slowly turn to different colors. Too long of the trapped blood for too many sets of constriction routine, you get darken skin as the old dead blood deposited in the tissues. Use warm towel/compress to massage the tissues. 15 minutes a day or palm massages after a nice warm towel wrap. Red light therapy also helps. It took care of our darkened skin due to intense routines.
Since you are using the HydroXtreme at high pressure without allowing the penis to prime itself to handle the intensity, it happens. But in due time, the discoloration will subside and return to normal. Mine went away half a year after I stopped all my intense routines.