Schema extracted from https://a.slack-edge.com/bv1-6-8cacda2/block-kit-builder.93ca53691de7cf9d4417.min.js
Probably the url/bundle will change every deploy so you might want to go https://api.slack.com/tools/block-kit-builder, open devtools and search for that file (block-kit-builder) in source tab. Pass some unminify tool and search for "JSON Schema".
PS1: Some block/element types simply does not work, idk why.
PS2: If you want to explore easily, copy the code and paste to the devtools console.
Anyone landing here, the currently linked version is truncated but the first uploaded revision is complete:
https://gist.githubusercontent.com/renatorib/1fb1a9bd71435b41bee602d15bc56899/raw/f3a759f1d948a27a648f1739f3f6436440f01e95/schema.js