> isn't git effectively a blockchain?

IMO no, this is one of those cases where a word means a lot more than just its component parts, much like how "television" means a lot more than just "anything where you do far-seeing."

The practical/effective distinction between what is promoted as "blockchain" and older stuff--like "a distributed database with cryptographic features"--involves:

1. Unrestricted global public membership, new nodes can be created by anyone at any time.

2. A global data structure that cannot have more than short-term conflicts or branches. (Hence a pruned "chain" instead of "tree".)

3. A cascade of other features all designed to stop someone from taking advantage of #1 and #2 to take over with an infinite army of sockpuppet nodes. (Proof of work, proof of stake, etc.)

In contrast, with git:

1. The default membership is "just me." Groups are ad-hoc as you find other people who you do (or don't) agree to work with and choose what to push/pull to one another.

2. Separate branches are extremely normal and can live indefinitely, and it's also normal to shift data from a conflicting branch or entirely rewrite subtrees.

3. There is no "majority rule" or "tiebreaker" logic, and thus no extra machinery to try to stop it.