I wouldn't wear blue jeans and black ankle boots in the same way that I wouldn't wear mink tights with flat lace-up shoes like my lovely nan. It's no big deal, it's very unimportant in the grand scheme of things and saying it's a fashion faux pas is not being bitchy it's just stating a fact. I certainly wouldn't judge anyone by their clothes. I'm really no goddess of fashion myself, I am sitting here in my partners huge jumper, my old jeans and slippers- that's hardly fashionable now is it?
To answer the question wtf can one wear with jeans? Well, it depends on the jeans.
Baggy, boyfriend jeans= funky trainers/pumps or sandals, maybe even chunky boots like Wranglers or Timberlands.
Skinny jeans= calf length boots, uggs (though mumsnetters generally hate them!), heels. My sister's look nice with flip flops but I would look like a golf club.
Bootcut jeans, I am a lover of sandals or ballet slipper type pumps but you could wear high heeled brown boots if they were modern looking or like cowgirl boots maybe. Just not Clarks black ankle boots unless you don't mind committing a fashion faux pas.