Hard to say without a scale to measure, but they are SMD leds, so measure the length and width and look for a SMD blue one of the same ( or similar, 0.2mm difference is close enough to solder still). Only issue you might have is that the forward voltage drop of a blue led is higher than the green ones, so they might not light at all if the original design had 4 green in series to make a voltage drop of around 9V, the same 4 blue will drop around 14V and may not light. Hard to say without seeing the back of the board.