My discoloration was the same way, when soft on the shaft almost the whole underside was black/purple. NOT PRETTY. This is essentially the way to turn back time to go back to pumping. I know I want to eventually. But yes you are right. I did not sign up for this either nor was warned.
I think from what I have read so far is that when you are erect and full of blood. The pump doesn't know one thing from the next, so extreme pressure is exerted on your dick, therefore causing blood esentially to rush to the surface under the skin. So the blood essentially gets trapped there in the skin, but acts somewhat as a permanent looking bruise. Atleast in my case.
I am trying to think of something to compare it to easily but I can't.
(I guess like when you have an unpeeled orange or some fruit and squeeze it, the juice comes out from the inside but stays under the skin, too much pressure. KABOOM! skin peels and juice comes out LOL ouch, but our hands can't pump that much so no worries there.)
And I usually do it at night so I have no time for a shower after. 
