There is no block hash in get block api response

General
2

This is available in the certificate of the block that you can get by querying the block as msgpack (?format=msgpack).

In JS:

const algosdk = require("algosdk");
const base32 = require("hi-base32");

const c = new algosdk.Algodv2("", "https://testnet.algoexplorerapi.io", "");

(async () => {
  const blk = await c.block(13819291).do();
  const blk_hash = base32.encode(blk["cert"]["prop"]["dig"]);
  console.log(blk_hash);
})();

Other SDKs work similarly, the client methods are: GetBlock for Java, block_raw for Python, BlockRaw for Go.