[tjchear]

Blueprint describes the thing that is to be built. API describes how two parties talk to each other.

I'd say API is closer to a protocol or a contract than it is to architectural blueprint.

[sjg007]

Well a blueprint communicates from the architect to the builder/carpenter/plumber etc...

[Twisol]

Then the blueprint is the contents of the communication, not the method of communication itself.

The blueprint would be communicated by mail or sneakernet.

You can copyright the contents of an email (indeed, I think they have an implicit copyright, don't they?), but you can't copyright the way in which emails in general are transmitted and exchange. (That would probably be a matter for patent.)