This is more common than you think, and basically it's all down to shyness.the more you worry about it, the more likely it will happen.I know on certain occassions I have suffered from it, especially when I was younger.try to remember that when you blush, the other person wont even notice.the more you worry, the more likley you will blush.try to take deep breaths, and relax basically.the blushing itself only lasts a few seconds at most.for some people it can even be classed as a phobia-erythnophobia.It's all to do with social anxiety , and it can be beaten.think of it rationally, what if you face goes red?who cares?If you take this attitude, you will revert back to normal blushing, which happens to all of us at some time or another.good luck!