The theoretical probability of landing on blue is calculated by dividing the number of favorable outcomes by the total number of possible outcomes.
In this case, there is 1 favorable outcome (landing on blue) and 4 total outcomes (red, yellow, blue, and green).
Thus, the probability is:
\[ \frac{1}{4} \]
So, the theoretical probability that the spinner will land on blue is \(\frac{1}{4}\).