Since there are two blue blocks and four total blocks, the probability of pulling a blue block is 2/4 or 1/2. Since you replace the block each time, the probability of pulling a blue block remains constant for each trial.
Using the formula for the expected value of a binomial distribution, where n is the number of trials and p is the probability of success:
Expected value = n * p
Expected number of blue blocks pulled in 12 trials = 12 * (1/2) = 6
Therefore, you would expect to pull 6 blue blocks if you try 12 times, assuming the blocks are replaced each time.