Someone cleverer than me will explain it better, but a bitcoin isn't anything other than something that enough people have agreed has value. It's like trading in an IOU that can't be paid, but everyone agrees has value. I some respects that's like a fiat currency, but ultimately they're backed by a government. They can (and do) default too, of course. But that has repercussions.
The blockchain (and the encryption) isn't to do with bitcoin, per se, it's the mechanism that makes bitcoin tamperproof, otherwise I could just claim that yep, I have a pile of bitcoin. The ledger proves whether or not I don't.
There are multiple blockchains, it's just that longest wins out – that's where the work comes in, as mentioned earlier.