I thought it was a myth as well, until I experienced it.
It won't happen under normal circumstances, meaning I can go weeks (literally) without as much as masturbation with no problems.
However, one day I was chatting online (sexual in nature) for hours and had gotten really horny, but I didn't cum. I watched quite a bit of porn later without jacking off. Why didn't I j/o? I have no clue. I was super super horny, like rock hard the entire time and this must have lasted an hour or two. I watched 2 or 3 porn segments (~30 minutes) straight through without touching myself beyond a few quick strokes. Again, I don't know what possessed me to act this way.
The blue balls actually hit like an hour after. I wasn't doing anything sexual in nature, wasn't particularly horny and was just surfing the net. A dull ache developed in my scrotum and developed into a very intense pain in my balls and in the area between my balls and anus (taint area..lol). It was a really painful and heavy sensation. Felt almost like I had to take a dump.
At first I had no idea what was going on (never happened to me) and embarrassingly, for a few minutes I thought I was actually having some type of medical situation.
By this time the pain had gotten really bad and was actually starting to scare me.
"What the hell is going on?!?!"
Eventually I figured out what was going on and my thinking was that the only way I could relieve the pain was to j/o and ejaculate. Let me tell you one thing, if you are not hard when blue balls strikes it can be difficult get yourself "up" with the pain. I had to "work through the pain" like an athlete or something. It was one of the only times when masturbation felt more like work than play.
Anyway, after I had cum, the pain immediately lessened but definitely didn't go away completely. I crawled in my bed in the fetal position (yes the pain was that bad) and ended up falling asleep. When I woke up everything was fine.
So yeah, I now fully believe in the existence of blue balls and hope to never experience them again.