Whichever machine has the NAT's external IP assigned to it will accept or refuse the connection, depending on whether they have a server running on that port or not.
It's not like I'm sat here thinking "I know it does block traffic, but I'm going to lie to everyone that it won't". NAT in fact, actually, really and honestly, doesn't block traffic, and I think I've been pretty consistent in saying as much.
A stateful firewall would block packets addressed to the router, or to machines behind it. NAT not translating a packet won't do either of those things.