Function get_alt_block_hash 

pub fn get_alt_block_hash(
    db: &BlockchainDatabase,
    block_height: &BlockHeight,
    alt_chain: ChainId,
    tx_ro: &Snapshot,
    tapes: &impl TapesRead,
) -> DbResult<BlockHash>

Retrieves the hash of the block at the given block_height on the alt chain with the given [ChainId].

This function will get blocks from the whole chain, for example if you were to ask for height 0 with any [ChainId] (as long that chain actually exists) you will get the main chain genesis.