Image one is a post on Write.as. Image two is the exactly same post, but on a WriteFreely-hosted blog. Neither blog has custom CSS.
They both display normally on the main blog page; it’s only on the individual post pages that it goes wonky, with this extra space between paragraphs when in a blockquote
(Other posts with multiple paragraphs within a blockquote are the same: correct on Write.as, incorrect on WriteFreely.Host.)