Hey, you blankety-blank! How in the world did blankety-blank end up as a euphemism for a whole range of taboo words? It goes back to the nineteenth century, when even words like damned and hell could only appear in upstanding publications when bowdlerized with dashes. Those printed dashes in the place of words or letters were called blanks, so it was a funny way to euphemize your speech by verbally inserting blanks where the naughty words would go.
Blank works as a euphemism for a punchy monosyllable. Other variations — blanked, blanky, blankety — developed as euphemisms for taboo adjectives. The OED suggests that blankety was used to self-censor bloody. Perhaps it was, once, but then how did it gain an extra syllable? Why not just use blanky for bloody?
I would conjecture that it’s because blankety simply sounds better than blanky, regardless of what adjective it might have once been expurgating. That’s especially true when accompanied by a blank or two. Blank blank blankety blank. Blankety blank blank. It sets up a punctuated rhythm that is appropriate for obscenity, even obscured obscenity. The rhythm is a bit like hail falling on a roof, or perhaps a hail of bullets.