Assuming the chances are equal to the recorded ratios of past recorded plants, we can assume that the flower has a 550/1200 proportion chance. Out of every 1200 plants, 550 bloom as blue. Reduce this to 275/600, then to 55/120, then to 11/24.
Your answer will either be a fraction (11/24), a percentage (~45.8333333%), or a decimal (0.458333333).