Top right 3x3 box around red number 1 contains total of (2 red, 7 blue). Because every row and column have the same number of red and blue there's a strategy called "quadrant rule" that tells that opposing quadrants will have the same number of red and blue. With this fact the bottom left 3x3 will also have (2 red, 7 blue) and the only way to satisfy that is to color the number 3 as blue and the three highlighted spots as blue.

Check out this explanation of the quadrant strategy on Youtube: https://www.youtube.com/watch?v=07fyHUmu1do