Being of darker complection, I can't really say that mine have ever turned blue, but it is indeed real!!!! In my experience it feels exactly like when you get hit in the nuts, except the feeling never goes away until you ejaculate.
For me I used to get them after spending time with a previous GF, we weren't really doing sexual stuff, but i was so into her, the first time i got it, i thought something was seriously wrong, idk what made me think to go jerk it. When i came it HURT, it felt like someone literally squeezed the cum out of my nuts, and it burned coming out. The volume was easily 4x what i'm used to. I'm not a cum shooter, but i am when i have blue balls. It was more painful than satisfying. In other situations i have to jerk twice to cum, the first time its like a painful retrograde ejaculation, the whole burning/squeasing sensation comes, then i can jerk right after and then the cum really flies.