What makes many water soluble fertilizers blue?

I've noticed that water soluble fertilizers tend to be blue. What chemicals or chemical reactions cause this color here? Is this necessary or for aesthetic purposes? I haven't seen this in organic fertilizers. I figured chemists might be able to examine the ingredients of Miracle Gro or something and tell me. I'm guessing it has something to do with copper.

I have some Miracle Gro on hand, the water soluble kind: The blue stuff is copper sulfate, or ${\rm CuSO}_{4}$. Here is the MSDS for Miracle Gro Water Soluble All Purpose Plant Food, which lists the chemicals in the mixture.