Bluish paper

Bluish paper (French: bleuté) can be very strongly to very lightly colored. It is a vague enough term used to describe different stamps so that what you find on a particular stamp may not be what you think.

General
To be obvious, the bluish color must go all the way through the paper to the back. Printing of a blue or ultramarine stamp can leave a residue or "cast" on the front surface to make you think you have a bluish paper. You do need to check both sides.

Some stamps are slightly bluish and can be hard to tell from white papers. Examples are the Great Britain 1882-83 shilling and pound values and the Barbados 1897-98 Diamond Jubilees. Viewing the back of the stamp against an orange background should prove useful in detecting those bluish papers.

US Washington-Franklins
In 1909, the Bureau of Engraving and Printing continued its experiments by introducing rag content/cotton fiber paper for then-current flate plate Washington-Franklin designs. This gave the stamps their characteristic GRAYISH color slightly tinged with blue. The higher quality also allows further identification of this bluish paper since the double line watermark is quite clearly visible when held to the light, not typical for "normal" double line watermarked stamps.

This paper color can be hard to tell. 10c yellow stamps can often look like bluish paper stamps, 13c blue green bluish papers can look like normal paper. The most obvious example is the 1909 3c Lincoln on bluish paper because of the color contrast. Viewing the back of a stamp against an orange background may be useful to see the color. Again, an important test is holding it to the light. There are other grayish paper stamps including the so-called China Clay paper stamps, but those will fail the hold-to-light test.

Washington-Franklins with truly blue paper are something else altogether. The 5c blue stamps can have a (true) bluish cast to the paper, but this is from the printing process. When flipped over, the backs will not be blue – it's not the paper that is blue. There are forgeries dyed in blue ink that will fool only inexperienced collectors. If the perf tips are darker, it's a clear sign that the stamp has been dyed, aside from the incorrect color.

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