Earlier this week I threw a few handfuls of safflower & black oil sunflower seeds on my back patio for the juncos and who ever else might be interested. A squirrel came by and ate most of the sunflower seeds but appeared to ignore the safflower. However, the next day there was another squirrel (or maybe the same one) eating the safflower seeds. So, I'm guessing it's simply not their "favourite" but they'll eat them if they are hungry enough!
-kris