When you add the blockquote render hook "render-blockquote.rss" and don't provide "render-blockquote.html", HTML rendering changes: "render-blockquote.rss" replaces the default HTML blockquote rendering. I assumed it would only affect RSS and it messed up my site. IMO this is counterintuitive.

What version of Hugo are you using (hugo version)?

$ hugo version
hugo v0.140.2+extended+withdeploy darwin/arm64 BuildDate=2024-12-30T15:01:53Z VendorInfo=brew

Does this issue reproduce with the latest release?

Yes.