Part of the issue is that blue tarps are made only for temporary use and will fail long before better designs. TarpaFlex makes most of the economy tarps you see in the U.S., and here's what their website says:

"Blue tarps are the tarps that most people come into contact with the most, and they are excellent for common use because they are so very inexpensive. Blue tarps cost just a mere fraction of the price that the heavy duty white tarps may cost, but that is because they are not meant to last a long period of time. In most cases, blue tarps are used just a few times and are easily replaced with just a few dollars. The downside to having traditional blue tarps rather than heavy duty white tarps is that they absorb sunlight and weaken; they easily tear and are not good for transporting or covering and pulling extremely heavy, pointy, or otherwise difficult objects."

Blue tarps are proving to be insufficient in the ongoing relief efforts in Haiti. Bazillions of them were sent after the earthquake and have been put to much harder use than they were designed to handle. The blue tarps are disintegrating under the tropical Haitian sun and are holding up poorly as the rainy season hits.

I'll bite. Why not? All I can come up with is "because the food odor would attract them", but I assume that's not it, since using a clean one takes care of that.

Your first guess was right -- it's the food odour issue. I blanch at the thought of waking up with a bear in my face.

IME, it's hard to clean dishes as thoroughly in the bush as would happen in your dishwasher at home. Plus, many plastics will hold the odour of food/spices even after washing.

In terms of sensitivity to minute smells, think of a bear as a tracking dog. You just can't be too careful.

blue tarps..was there such a thing as the "old blue tarp" that was made much better and what we see now is a knock off?
i sort of recall seeing many years ago huge hay piles covered with blue tarps that were held down with ropes weighted with old tires.they seemed to last all fall and winter here in Minnesota.
thinking about it i used a blue tarp as a canoeing shelter back in the early 80's and it did seem much thicker than the ones i cover my boats with in the winter,you can just about see thru the new ones and they only last a season at most..the brown tarps arn't much better.

I have lots of blue tarps around my house. Unfortunately several are used to keep pigeons from pooping on gear in my barn (wife won't evict them ... argh!!!). Some are clearly thicker than others, though I haven't kept track of where I've purchased them from and they don't have labels on them to allow some kind of backtracking.

The different colors seem to relate to thickness - blue=thin, brown=thicker, silver=thickest ... or something like that.

I even bought a couple of the bright orange plastic tarps just in case I need them.

When I was a kid my dad bought a bright orange nylon tarp/fly, but I've never found an orange one on-line. When I bought my Tundra Tarp for camping I caved in to the mute color green.

Oddly enough with the rainy weather, I have been researching tarps for a primitive trip I'm planning this fall. I have some blue and brown tarps I bought from the local megastore to line my garage walls for a graduation party, but they are too thin and wimpy for wilderness use. I was surprised to learn that there is a wide variance in the thickness and weave of tarps that look alike, and are close enough in price. The local mega store sells blue tarps that are 3.5 to 5 mils in thickness. I found many sources for inexpensve tarps that were 8, 9, and even 10 mil or more. The color seems to have some relationship to the thickness, at least in the sizes and price ranges I was interested in. Blue was the thinnest, orange was 7-8 mil, forest green was 9-10 mil, and silver and white were even thicker. Tarps have fibers that criss-cross in the middle of two laminations of plastic. The more fibers per inch, the stronger the tarp. A 10x10 weave seems to be a good number to shoot for in a budget tarp. I really wanted a dark green canvas tarp, but after comparing the weight, prices, and specs, I guess I'll reluctantly go with the 9 mil forest green plastic.

Blue tarps are OK for routine use around the home, but for wilderness use I really prefer sil=nylon or the slightly heavier nylon tarps. They are adequately strong with reasonable care and are very light. Naturally, they aren't cheap, but that just means an old miser like me will use them carefully....

Interesting to know about the variation within the "blue tarp" category. I had thought they were all about the same.