Thank you for the question.
The Nemeth Code tells us that "In work arranged spatially for computation, only the general omission symbol may be used in braille regardless of how the omission is denoted in ink print. In addition, the number of general omission symbols to be used must be the same as the number of omission signs which occur in ink print. (section 58 of The Nemeth Braille Code for Mathematics and Science Notation, 1972 Revision, 2007-2015 Updates, emphasis added)
Since the print reader is not getting indication of how many digits or symbols to put in a blue rectangle, it would be inappropriate to determine how many digits and print signs would make up the correct answer and then to place the corresponding number of omission symbols in the braille.
Instead, I suggest using one general omission indicator for each blue box, right aligned in the spatial arrangment, and including a brief transcriber's note with these examples (for example, "In the following problems, blue rectangles of different sizes indicate areas to be filled in. In braille, a general omission symbol is used on each line to be filled in.")