What does it look like if a person has been blocked?

I have a family member that I'd like to block from texting/calling via my iphone. However, I'd prefer them not to know since they can be extremely vindictive. What does it look like from there end when they send a text and it doesn't go through? Does the message itself show red instead of green? Or does it just say "undelivered" (or something to that affect)?

NOTE: They are much older, find Facebook too complicated, and has a history of hard heavy drug use, so I don't think they'd see "undelivered", since it takes mental control & focus to read. But they'd see the change of color.