By definition, it is not a black swan. It is something that has been foreseen by the markets, and current valuations include the idea of a Brexit. If Brexit happens, the markets will go down, a little. If it does not happen, they will go way up.
Stick to the plan. That's for this kind of situation that I'm a proponent of gold (as a European citizen, I have a UK-heavy portfolio, my stock allocation suffered, but my gold allocation compensated the losses, so my net worth actually grew up these last days, and I've been able to buy cheap stocks). But now it's too late to consider it. You had a plan, now would be the worse time to bail out.