As a rule, no; ISPs don't block communication between peers within their network. Though also as a rule, home Internet users don't allow any inbound connections, so the lack of connectivity is impossible to test without the cooperation of someone else on the same network.
Also, communication between subscriber peers is sometimes a part of the network that isn't well tested. So for example, they may assign you a /24 netmask which reflects that fact the address is a member of a network of the appropriate size. But then if the underlying technology doesn't broadcast ARP packets between all the nodes sharing the same prefix, then such a configuration would be incorrect, and the result would be a lack of connectivity only between peer nodes.
If, on the other hand, they assign you a /31 netmask as you suggested in your comments, then in that case they're correctly accounting for the fact that the technology won't handle inter-node broadcast traffic. In that case, all traffic must be routed through the network's local router.