There are a number of questions people will ask you before we can help, so I may as well start.
1) When did you first start PE? That is, how much of a newbie are you?
2) How tight is the clamp?
3) What are you using to clamp, is there any padding?
4) Has this happened before? If so, how many times?
5) Was your penis cold or normal during the clamping session?
6) Did the pain in the base of your penis go away after you removed the clamp?
7) Did your penis at any time have a tingly feeling, or go numb?
8) When you removed the clamp, did the blackness in your penis go away? If so, how long did it take?
9) Which thread did you read that told you newbies should start 2x10mins clamping sessions (I assume daily)?
There are more, but those questions will probably do. From the sounds of it, your clamp is too tight or you may have damaged your penis. I don’t know a huge amount about clamping, but I would suggest starting with maybe 2-3 minutes a day and build up slowly from there. 2x10 minutes daily seems like a large amount of time for a newbie.