JOHANNESBURG, July 14( Reuters)-The South African rand weakened on Friday, bringing this week's rally to a halt as prolonged power blackouts and expectations that the central bank will leave its main interest rate on hold next week weighed on the investor mood. The risk-sensitive rand has been a big beneficiary of data this week showing cooling U.S. inflation, which has stoked speculation the Federal Reserve could pause its interest rate hikes after this month and driven the dollar to its weakes