Both Kieffer and Orient are rated as strongly fire blight resistant so would guess it is more likely to be frost damage. But that opinion might change if you continue to see this developing.

Well if it’s fireblight you’ll know soon enough as it will move down the branch. Blossom blast is another thing that happens in cool, wet weather. The 25 degree day could have done that I wouldn’t be too worried just keep an eye out on the trees over the spring season

I'd remove that "burnt" blossom cluster and any others like it asap, it won't produce fruit and if it is FB it will spread very quickly to the stem/branch.

After further reading, I'm leaning towards blossom blast. It seems freezing temps and frost injury provide an entry point for the bacteria to enter, especially affecting the bottom of the tree which is in my case. I don't see any "oozing" that might indicate fire blight. The other nearby trees were not were not yet in the same advanced stage of blooming so I don't think they were injured by the unusually low temps. If only the tree had waited a few more days. From what I've read, a copper spray which I should have done probably and a couple of sprays with streptomycin *MAY* have helped but it's too late now. Lesson learned, pay attention to low temps around flowering time. It's a wildlife tree anyway so maybe there will be some fruit yet.