The domain whois record for both blackra1n and limera1n shares same record and confirms that it has been registered by geohot on March 27. Whois record also reveals that it was last updated on April 13, almost effectively the same date when geohot successfully jailbroke iPhone OS 4 which makes it even more authentic. Though there is no confirmation about this by geohot himself.
If you head over to the site you will see a raindrop that looks just like the icon for blackra1n but with a lime color to it.
It's unclear if geohot plans to use the limera1n name for the untethered 3.x jailbreaks or for OS 4.0.