Blossom is a common, widely understood ingredient like sugar. Nostr apps would be anything you are making in the kitchen. Although there are other sweetener options like stevia, splenda, neotame, Xylitol, nobody knows most of that crap, and they want to use something that works and that everyone uses. :)

Basically it is a common media hosting protocol that makes it very easy for any client to use a Blossom server, and anyone to create a Blossom server and have integration directly with their nostr clients.