Sling cloned Crave. Sling's slapdash 'market' is probably built upon Crave's stealth messaging system. The stealth messaging that was developed by... the Shadow developers.
No, Sling's market is a blockchain, not a messaging system.
Shadowcoin's messaging system uses an ECC key for encryption (secp256k1) with a known trap door and is labelled "unsafe" on this site
http://safecurves.cr.yp.to Sling also uses secp256k1 for messaging but that will change soon. I am think of changing it to use Curve383187.
The crypto signing aspects of secp256k1 does not have a trapdoor so all these transaction blockchains are still "safe".