Post by Janis PapanagnouI just confirmed that game behavior by a test in explore mode.
(The earlier post would have been clearer if you had mentioned
that your shield at the time was not a shield of reflection.)
If wearing a shield (other than reflection), disintegration will
destroy it but you will survive. If not wearing any shield, but
wearing a suit (even leather jacket, and presumably a robe in
slash'em where it is worn in the suit slot rather than the cloak
slot), disintegration will destroy your cloak, suit, and shirt but
you will survive. If not wearing a shield or a suit, you will be
disintegrated.
I don't think it's possible to get a double bounce and triple
hit against the hero on the same breath attack but I might
be wrong. I don't think the range can be long enough to
hit the hero and pass through, bounce off an adjacent wall
or reflecting monster, hit the hero a second time and pass
through, then pass through the blank dragon, bounce off
the wall behind it and pass through it again to hit the hero
for a third time. However, it might be long enough if the
dragon had been wearing an amulet of reflection while in
some other form then kept that when being polymorphed
into a black dragon. The distance for the second bounce
and third hit would be shorter that way, both by a pair of
steps and by not passing through the dragon.