Is it possible to DDOS a blockchain domain?

DDOS is not possible on blockchain domain lookups if you run your own node because you have your own copy of the blockchain and can read it yourself.

If you are referring to DDOS on the web content - not on the name lookup - assuming it is stored on a p2p service like IPFS you can store the content locally by running your own IPFS node again preventing DDOS.

If the content is instead hosted on amazon then yes the content will be subject to DDOS just like all other web content and you’ll want to use cloud flare or other providers to help mitigate those problems.