Where this blob content should be saved? In Peercoin blockchain ?
No, blockchains should not hold data. It can be save externally: ipfs, http sever, torrents.
So you could, for example, store a hash of the Peercoin code itself, and all revisions. The Foundation would form a multisig as the ‘owner’, and you could use this PA to verify your downloaded version of the code.
You can attach this to normal PPC payments, right? So you could attach a contract to a payment, then update that contract later with amendments?
Contract - a digitaly signed PDF file (Good for print)