I presume "blue square" was a misstatement, as there is no square here. In fact, there is no rhombus in the picture (the diagonals are not shown as perpendicular), perhaps to hide something from you. So try drawing a more accurate picture (though that will not in itself make the answer obvious).

Here's one approach, which may be in part what you meant in mentioning a square: One possible quadrilateral that could generate this rhombus would be a rectangle; **if the problem has an answer**, the answer would have to be the area of that rectangle. What is it?

But in order to really answer the question, you need to convince yourself that the blue area is **always the same**. I see at least one very nice way to show that, involving triangles congruent to the four triangles outside the rhombus.