If you black a contact on iPhone 6, you won't receive any phone call, message or FaceTime from that number / contact. To block a number / contact on iPhone6, iPhone 5 or other iDevices, do as follows:
From the Call Log: This method is useful when you haven't saved the to-be-blocked number to your contacts
1. Open the Phone app on your iPhone from the bottom of the home screen (if you haven't changed its default place)
2. Tap the Recents button on the bottom of the screen
3. Find the number / contact you'd like to block and then tap the i icon right to that number/contact
4. Now, scroll all the way down to the Block this caller option and tap on it
5. Tap the Block Contact button to confirm you no loner want to receive any phone call, message or FaceTime from this person.
From the Contacts
1. Open the Contacts application on your iPhone
2. Find the Contact you want to block and tap to open the contact's details
3. Scroll down to the Block this caller option, tap on it and then confirm the Blocking by tapping the Block Contact button
See Also:
How to Block Just one Number from a Specific Contacts
How to Unblock a Contact on iPhone