I'd say that it's only quasi centralized. At its core, keybase is a directory of signed statements posted on social media. Where space permits, these statements

I'd say that it's only quasi centralized. At its core, keybase is a directory of signed statements posted on social media. Where space permits, these statements include the person's public key. Verifications can be checked directly on Facebook, Twitter, etc. without using keybase, somewhat decentralizing the service. As you mentioned, writing to blockchain pulls keybase further out of the equation. They're more of a facilitator for other applications -- Signal is a packaged application itself.