https://en.wikipedia.org/wiki/Blister_pack is actually what the example given in STYLE-399 would be.  https://beta.musicbrainz.org/doc/Release/Packaging already defines clamshell as having a hinge, which blister packs do not have.

Example: https://www.sonymusicshop.jp/m/item/itemShw.php?site=S&ima=4151&cd=MATO000004627 is explicitly named as a special blister pack package.