1. Only use blockchain as entropy source when the cost to manipulate blockchain is higher than it's "reward"
It's worth noting that you can increase the cost of manipulating the blockchain. For instance I used to run a lotto, where I'd use a bitcoin block hash to determine the winner. However, because the prizes were reasonably big compared to the blockreward it would've been (very) profitable for a miner to manipulate. So I stretched the hash with an operation that might take like 20 minutes or something.
That made it pretty impractical to manipulate the lotto (after finding a block, a miner would have to wait 20 minutes to check if it was good or not), unless the miner was prepared/able to 51% attack bitcoin at the same time
2. Use many blocks in a sequence as entropy source since it's difficult to manipulate n blocks in a sequence.
How does that work? If you get n blocks in a sequence -- the result is always going to be influenced by the last block in the sequence, who would be able to perform the same sort of attacks as if you just had used that 1 block.