My question is if the Wallet.Dat file on my computer (from the QT client install) were ever compromised, could that be used to steal my BTC from the blockchain?
Note that there is no such thing as "stealing Bitcoin from the blockchain". Bitcoin cannot be sent to or from the blockchain, that is not how Bitcoin works.
To answer your question, if your wallet.dat were to be compromised, your Bitcoin will be safe assuming that you have properly transferred your Bitcoin to a Ledger Nano S. By properly I mean that you generated a seed using your Ledger Nano S and creating Bitcoin transactions from your Bitcoin Core wallet that sent your Bitcoin to addresses from your Ledger. If you did that, then if your wallet.dat is compromised, your Bitcoin will be safe. The private keys your Bitcoin are associated with are on your Ledger, not in your wallet.dat file.
Of course Bitcoin sent to addresses from your Bitcoin Core wallet will still be in your Bitcoin Core wallet and not on your Ledger. Those coins can be stolen if your wallet.dat is compromised.
Even with a new wallet, it is advised that you hold onto your original wallet.dat and keep it in a secure location. You never know when you may need those private keys; someone could send you Bitcoin to an address in that wallet.