Because there is no blue colored food in the nature. Blue color can easily determined in the case of contamination inside of the product/process.
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Hi Honour; Blueberry is not actually blue. As you can see from the attached picture blueberry has navy blue color around of it and when you mash it,it becames purple ! 
Thank you very much for your information @johnfound
as a different point of view, maybe blue is one of the few colors that even color blind people (especially the green-red type ones) can discriminate from the other colors in every light conditions. I know from myself 
As it is stated earlier, bluish colors are not common for foods. Considering almost 10 percent of the population is color blind, this point might be important.
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Dear Gökhan…very good information i did not think in that way…
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There is still no specific legislation on the use of Blue-colored gloves in the food industry, but for a Good Production rule, blue-colored gloves are preferred because Blue is the least present in food. Some factories have spectrograms for detecting such color objects on the production lines.
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