Something I never given much thought, but recently got more interested in is the use of blue denim by the military.
I was looking through a book on WWII gear I have and came across a couple of pics but without any further explanation.
A google search yielded only one result.
Apparently from early till 1900's the beginning of WWII military work clothing was made out of blue denim.
Paul's book mentions it, but only briefly and mostly goes into who produced it.

Here's is what I found so far:
(these are the 3 images I got from the book I mentioned, I looked through it again, but these were really the only denim pieces)


The following is taken from the only informative website I could find on it, here:

History of the Army Blue Denim Work Uniform


Blue Denim work clothing was adopted as Standard by the Army on 11 June 1919, replacing brown work clothing used before. The top was a jumper style pullover, the trousers had five pockets -- two front, two hip, and a watch pocket. In 1933 a one-piece work suit (coveralls) was adopted in blue denim for use by mechanics, drivers, machinists, and others in similar roles. This was in addition to and did not replace the two piece work uniform. These blue denim coveralls were used until replaced by herringbone twill (HBT) one piece coverall in 1938.



The Army Blue Denim Work Uniform of 1940
In 1940, after field complaints about pullovers ripped down the front, the jacket style of blue denim work uniform was adopted. It had two large front pockets toward to bottom of the garment and buttoned up the front. It was made to be closed at the collar.
At the same time in 1940, the blue denim trousers were improved by using a style made from the same pattern as the khaki summer uniform trousers, with the front pockets moved to the side. The M-1937 "Daisy Mae" style hat was also provided in blue denim (with white stitching visible) to complete the uniform, as seen in the photo at left, made in 1940. These blue denim garments were used until replaced by the HBT two piece field uniforms in 1941.
Although the replacement was standardized, the blue denim uniforms continued in use for a few more years as supplies were used up. Since the great increases in supplies for World War II came after the HBT uniforms had been adopted, blue denim garments were relatively rare and are not much associated with the Army in World War II.


Blue Denim in the U.S. Navy and Marine Corps

Prior to World War II the Marine Corps also used blue denim fatigue uniforms, similar to the Army pattern but with a two-piece bib overall and jacket design. They also adopted a one-piece coverall. All USMC garments had "USMC" metal buttons. As with the Army, the USMC blue denim work uniforms were replaced in the early 1940s by sage green HBT or camouflage utility uniforms.
In 1901, Navy regulations authorized the first use of denim jumpers (shirts) and trousers. Regulations were changed in 1914 to allow the dungaree outfit to be worn by both officers and enlisted sailors. Enlisted personnel in the U.S. Navy wore blue denim trousers for work duty throughout World War II, with long or short-sleeved blue cotton chambray shirts. Several shades were used, but the shirt was usually a lighter shade of blue than the trousers, and lighter than the Army or USMC utility uniforms. Most enlisted personnel on board a ship would be dressed in the denim dungarees with the lighter blue shirt, a tee shirt, or no shirt. The trousers were straight-leg, not bell-bottoms. In the photo to the right (ed: above), taken at Naval Air Base, Corpus Christi, Texas in August 1942, an aircraft mechanic is fueling a Navy plane dressed in the blue denim trousers and short sleeved chambray shirt topped by a swabbie hat.

Sergeant George Camblair on fatigue duty in his barracks, wearing blue denim work uniform. Fort Belvoir, VA, September 1942.

Use of denim in the navy seems to be more commonly known, or at least there are plenty of pictures to be found on which denim is being worn.