US President Donald Trump has placed the Caribbean island under a de facto oil blockade, straining its ageing power grid.Cuba has reported its second nationwide blackout in less than a week, plunging the island into darkness shortly before evening.On Friday, the Union Electrica de Cuba, the state-owned utility in charge of the electrical grid, announced that the outage began at 4:30pm local time (20:30 GMT).Recommended Stories list of 3 itemsend of listNo explanation was given for the blackout. But it follows a similar power failure on Monday, bringing the total number of island-wide outages to four since the start of…