Any synonym for blend will do but you really don't need a term because that's what straight bourbon whiskey is. The variations are single barrel and bonded. Calling it "mixed" bourbon or anything like that will just confuse things. It's bourbon.
Remember too that a straight bourbon can be all whiskey made at the same time at the same distillery, or at the same distillery but at different times, or even at different distilleries although, for some unknown reason, all of the distilleries have to be in the same state. No distinctions are made. They're all straight bourbon.
In the world of whiskey, "blend" is a term-of-art, a word with a specific meaning in this realm that supersedes its common meaning.