Cockroaches
In Blaise's mind, muggles heavily resembled cockroaches.
Pesky little filthy things that cropped up like weeds whenever there was the slightest absence of cleanliness, purity.
They fed off of anything, trash to fine dining. Whatever they touched was immediately rendered useless from the germs and disease they carried or needed to be cleaned through the use of fire and or boiling water treatments.
And the thing that made any well-bred pureblood citizen's skin crawl;
they just refused to die.
No amount of feindfyre, dementors, unforgivables or dark magics could eradicate them completely. Instead, they popped another dozen or so pustles of filth into the world and sent them off to ruin something else. Its kind of amazing really. Not even their own messes would do them in. Even the earth itself lost that battle. Freakishly strong quakes, wave and magma spilling over the earth resulted in squishing a few million at best.
They've proven themselves as perfect pests.
Blaise is convinced that when the world ends, only roaches and muggles will be left.
Blaise looks so far down on muggles, that he's actually sort of looking up to them.