The person whose number you've blocked won't receive any sign that their message to you was blocked; their text will simply sit there looking as though it were sent and not yet delivered, but in fact, it will be lost to the ether.

Can someone tell if you block them?

How to know if someone blocked your number on Android. If an Android user has blocked you, Lavelle says, “your text messages will go through as usual; they just won't be delivered to the Android user.” It's the same as an iPhone, but without the “delivered” notification (or lack thereof) to clue you in.

Will someone know if I block them on my phone?

First, we want to clear up any confusions you may have. The person you've blocked will never get any sort of official notification. Google respects your privacy and will not let the person knows that they've been blocked.