Plugging in 43 as the hidden number satisfies both of those requirements, meaning our hypothesis was correct, and the Blue number is 43.
The difference between 31 and 33 is 2.
Our Blue numbers are 31, 33, 37, ?, and 51.
Since the last difference was 4, the difference between the third number and the hidden number should be 6, and the difference between the hidden number and the final number should be 8.