If I block someone on sprint, do they know they're blocked?

Do they receive a message that it won't go through or do they think I'm still receiving the text?

The text will not be delivered. They will probably guess why.

"Blocked" is normally defined as "delivered but silently discarded by the receiver's phone". In this case the sender gets no notification of the blocking.

If your carrier lets you do some blocking at the server level, then the sender may get a "not delivered" status code back.