To block a crossup, it is not determined by a random direction. To help explain a bit better, here are a couple of examples (most crossup situations happen because of an attack in the air):
If you use Grave’s Hurricane attack in the air (his air B, looks like he’s spinning horizontally) while you’re landing “on top” of the opponent, it can be hard to determine if he’s going to hit you from the left side or the right side. This is a crossup situation. You have to choose either to block left if you think he will hit you from the left side, or to block right if you think he will hit you from the right side. A wrong guess will get you hit.
Another example is Valerie’s BBB combo. The third hit of her B can hit you from the left side or the right side depending on if Valerie presses forward or not. If you choose to block her third B, then you have to guess if she will hit you from the left or right side and then block from the appropriate direction. This is an example of a ground crossup instead of the usual air crossups.
I hope this explained it a bit better! Basically, you have to either know by sharp observation whether to block left or right or you have to read the opponent correctly.