It's another frontend for Hive. Hmm, I'm not sure why it won't sign you in.. Have you tried installing Hive Keychain in your browser to sign in btw? Think I had that problem before even though I've never changed my keys. I personally like using PeakD better because it's easier to use but if you're into simplicity you can just use hive.blog.

Am I right in saying if you post on peakd, then it's automatically on the hive block chain.

Yes. :D