I guess it depends on what the upstream patches look like.  If they're always coded against release tarballs and not against git repos, then they'll assume (correctly) that Misc/NEWS exists.  In that case you shouldn't have to do anything.

If the upstream patches are extracted from GitHub PRs, then they'd presumably have files in Misc/NEWS.d.  You could just ignore them.

blurb doesn't support combining Misc/NEWS.d files with an existing Misc/NEWS file in a single operation.  But you could run "blurb split", breaking Misc/NEWS into constituent files in Misc/NEWS.d, and then "blurb merge", rebuilding Misc/NEWS from those constituent files in Misc/NEWS.d.  Naturally this would pick up the new files from Misc/NEWS.d too.

Currently blurb is designed for the CPython developer workflow, so operations like "blurb split" and "blurb merge" also automatically stage the files in git.  I can add a flag to suppress this feature, if it's going to become common that "blurb split" and "blurb merge" are run on tarballs.