If I block a contact on my iPhone, will they know I've blocked them?

If I block a contact on my iPhone, will they know I've blocked them? - 1

No
Unless they call you

Not but they will understand it indirectly… When they can't get you.

They won't get a direct message telling them that, but when they send you a message they may have a suspicion.

No. Thats such a great feature, I block spammers all the time.