My dog got a fright a few weeks ago. He got in the way of a runner who skidded to a halt in front of him kind of towering over him. Runner started running again and dog thought he was chasing him and ran off too. Since this incident he has started being reactive around cyclists and runners by barking at them. He never did this before. It seems to be any cyclist or runner who is dressed in black (the runner who gave him the fright was in all black).
What should we be doing to try alleviate his worries and stop him being reactive to them?