I doubt Netflix could get away with blocking an ISP, as it would be considered a net neutrality violation, and many of the countries Netflix serves have net neutrality legislation or regulation.
On the other hand, we may see Netflix and an ISP sign a deal to include Netflix access with all ISP customer accounts, much like Bell Canada signed a deal with Microsoft to use Hotmail for all customer e-mail (on Bell's own domain).
Net neutrality doesn't work both ways in the US. ESPN has already granted access to content to exclusive ISPs without a single legal issue. Netflix is already doing some of this by only allowing ISPs who participate in their Open CDN to have SuperHD.