Why are tarps usually blue?

From this site, because they are the cheapest, so they are the ones
most sold and seen:

http://www.tarpsonline.com/complist.asp

But why are they cheaper?

Hi peterfda,

I can take myoarin's comment two steps further, and maybe a Google
researcher can add to this and make a formal answer.

First, a quick search shows that the blue pigment used in polyethylene
products is cheaper than, for example, the green pigment. To make a
green pigment you need to mix the blue with yellow.

Here is a sample site for pigment prices:

The price of blue (Phthalocyanine Blue B) PVC, PE, PP pigment from
China is USD 4.93 / kg, and
the price of green (Phthalocyanine Green G) PVC, PE, PP pigment is USD 6.49 / kg. 
http://www.made-in-china.com/products/search/search_prod/6e6f2b6c696d6974/114/no/11/prod/5069676d656e74/Chemicals_Pigment.html

Second, the Phthalo Blue pigment does not fade it is completely
lightfast, and when mixed with other pigments makes predictable mixed
colours. So blue, especially phthalo blue, is perfect for a base
colour.
http://www.gamblincolors.com/materials/blues.html

Google Searches:
Blue pigment plastic products
Mixing blue pigments