Should I tell my partner that I cheated from a long time back?

Near the first few months of my relationship with my boyfriend wasn't going to well and thought it would end at any time. I did run across a one night fling which is not my kind of behavior but it did happen and never will again. Its been almost a year now and my boyfriend and I are doing alot better than I would have ever thought. I mistakenly told a friend who I thought was my good friend about it and now she is telling me I better tell him or she will.She is totally blackmailing me which I think is not acceptable. She gave me a dead line to tell him or she would. I personally think she is immature and this has nothing to do with her. I came to a conclusion that he shouldn't be told cause it will just hurt him and it is irrelevant to our relationship now. We are very much in love so why would I bring up a thing like that when it was so long ago? I thought at the time this event occurred it didn't matter to tell him cause I figured our relationship was about to break anyways.I believe if guilt is your only primary concern is making yourself feel better perhaps your partner is better off not knowing. Why should you hurt a loved one merely to absolve yourself of well earned guilt? Please give me all your opinions on what I should do?