The stamps being described as having bluish paper is unfortunate. It has led to generations of stamp collectors looking for stamps with blue paper. The color is, in fact, more of a dull slate grey. That is not quite gray, there is just a tiny. These are difficult to identify as many Washington-Franklin stamps when turned over look grey, as can be seen in the stamp shown above on the right.
In my opinion once you have seen a true (that is certificated) bluish stamp you will always recognise the bluish shade. I have illustrated above left the bluish shade, however different monitors will show different shades, on my wife's iPhone the stamp on the left is a kind of yellowish grey. And use it as a reference. This item is in the category "Stamps\United States\1901-40: Unused".The seller is "esteemedauctions" and is located in this country: US. This item can be shipped worldwide.