The answer is (b): the frequency will fall, because there is less heat radiation in the blue region, as can be seen in an mpeg video by clicking your mouse on the photograph below.
Although blue (and green as well as every other color) photons are more energetic than red photons, the frequency will decrease, because the thermopile is sensitive to infrared photons. This is a heat-sensing device, as demonstrated using Dan's head in Question #247.
