> We are also exploring ways for people to move easily between the languages.

That's currently impossible (to my knowledge) because API Blueprint encodes far less data than Swagger can. Moving losslessly between formats wouldn't work unless APIB changes...

> far less data

/author of API Blueprint here/ I would not say this is the case. API Blueprint focuses on different data for various reasons (design-first, documentation-orinented). In order to bring the Swagger support we had to introduce some extension in the format. We are, however, trying to unify the tooling under the hood of the refract project (https://github.com/refractproject/refract-spec/blob/master/n...) through (de)serialization plugins for the Fury.js library (https://github.com/apiaryio/fury.js)

I don't know anything about them.

Are they some kind of API mock-up tools?

/disclaimer: I work at Apiary/

A hosted mock server is part of Apiary suite of tools for the entire API design lifecycle. Others would include interactive documentation, traffic inspector, backend testing and style guide validation. We hope to encourage collaboration and bring the design-first, documentation-oriented approach to the world of API development.

And Swagger does the same?

Swagger and API Blueprint are two different formats to describe web APIs. Until now Apiary and its tools supported only the API Blueprint. This has changed today.