i can confirm it's on the Steem blockchain. The idea of a fork was considered but i do believe it's more easy (and still possible) to take some power on the Steem blockchain and impose certain enhancement and benefit from an existing community and supports
So you are going to have both steemit and busy on the same blockchain? How will that work?
Steem can have tons of apps build on the same blockchain, sharing or not the same content. You can filter the content you want to show, from any source/apps, or share the same content. Or you can also just ignore the content part and enjoy the use of the blockchain as a backend/database bootstrapping wallets & crypto with 0 transaction fee & 3s block time, for your app.
i think busy and steemit will just share the content at least for the blogging part.