The nitrogen bubbles in blood are definitely what happens when a scuba diver returns from depth too quickly, and becomes "the bends" - but it happens on a slower scale than this obviously (and from far shallower depths).
So yeah I can imagine this is probably somehow connected with being inside the pressure vessel and then abruptly being "at depth" as soon as it ruptures.
As for it boiling the blood, maybe that's what happens, but you wouldn't feel it surely, it'd be too quick.
A horrific way to die? No way, it'd be instantaneous; the blink of an eye. Remember the twisted bodies that were found after that explosion in Beirut a couple of years ago? The aftermath looked horrific but there's no way they felt a thing, too quick an end.