If you block a number on an Android phone, the person you have blocked will not be notified that you have done so. However, they may be able to infer that they have been blocked if they try to call or message you and cannot get through.
If someone tries to call you from a blocked number, they will not be able to connect to your phone and the call will go straight to voicemail or may be rejected automatically. If they try to send you a text message, it will not be delivered to your phone and they may receive an error message indicating that the message was not sent.