Air travel from Israel to Bulgaria is suspended indefinitely, and Israelis should also cancel any plans they had to go to Cyprus or Greece any time soon. Other than that the travel impacts should be minimal, though if you had flights into Burgas you're going to get diverted. That airport is going to be closed for at last a few days.
As for how this will impact Bulgaria's tourism industry, we're skeptical you'll see much of a change. None of the many underlying reasons to travel to Bulgariageneric awesomeness and fantastic locations and hipster dives and sexhave been changed by today's bombing. And it makes no sense to go to other countries instead of Bulgaria, since terrorists are apparently trying to murder Israeli travelers in those countries as well. And finally, as a rule, we always urge you to remember that the world has never been safer for travel, and that you can't predict where and when these kinds of attacks will happen. Travel where you want to travel.
That said, there's a high probability that the Israeli government will take the murder of their citizens somewhat personallythey tend to go all Israeli when these things happenand retaliate against Iran or Iran's proxy Hezbollah in Lebanon. Inasmuch as a massive regional war in the Middle East will affect, well, everything, we'll keep you updated.