As Bonneau et al. showed a Bitcoin block header has high min-entropy, approximately 68 bits. So, in theory you could extract randomness for your protocol or application from Bitcoin block headers, but bear in mind that blockchain's entropy is malleable as shown by Pierrot et al.
For example, if you run a lottery whose randomness is extracted from the Bitcoin blockchain, a miner could interfere with this randomness by withholding blocks whose entropy is not profitable for her. She might want to do this, if she is participating in the lottery and the prize of the lottery is larger than the block reward.
To deter such an attack you might want to use verifiable delay functions(VDF). VDFs make impossible to learn the randomness derived from the block before some time-out (say 1 hour), since it requires sequential work to compute, therefore massive parallelization does not help here. Furthermore once the output of the VDF function is calculated it is easy to verify the computation given a small proof. By applying VDFs even the miner does not know the output of the VDF before the time-out. Therefore there would be no more financial incentive for the miner not to publish her block, since she would forfeit her block reward.