When you block a person on your Android phone (Black List), you no longer receive calls, messages or texts from that contact. To Block contacts on HTC One do as follows:
1. Open "People" App
2. Tap and hold with your finger on the contact you want to block
3. Select "Block Contact" from the opened list
4. A confirmation message appears and warns you about not receiving anymore call, or message from the blocked contact
5. Confirm the process and the contact in now in the Black List
To Unblock contacts on HTC One, do as follows:
1. Open "People" App
2. Tap the 3 dot icon on the top right and select "Manage Accounts"
3. Tap "Blocked Contacts" to manage black list
4. There you should see the list of blocked contact
5. Tap and hold on the contact you want to unblock
6. Select "Unblock Contacts"
7. Tap "OK" to confirm the unblocking process