Yep, I have had this happen to me, but my case was due to the strobe not clamping down right (also first time in the water with the strobe.) The black stuff coming out is battery acid, and I must say I can't believe that it worked after that. If the strobe was brand new, I'd send it straight back to Ikelite, and say it flooded on the first time in the water. How do you know for a fact that that you pinched the O ring? Perhaps it was something wrong with the strobe seal that will be still under warranty. Mine was under warranty (as it was brand new), and they sent me a new one. I'd send it back to them asking "why did the seal fail"?
When I disconnected the battery, I saw that part of the O-ring was up and towards the center, with some grooves, and I think that's why the clamp was not proper.