The pattern is referenced in Junior Red Cross Activities Teachers Manual, American Red Cross publication #606, published on October 15, 1918. The Manual is a terrific resource for understanding how war work could be integrated into school work, starting even in the primary grades. Before getting into the specifics of the articles to be produced, the manual discusses how the schools' war work can be used to teach social responsibility and contribute to community service. (See Chapter V)
Thus, while sewing clothes for refugees was incorporated into home economics instruction, it could also be used to teach geography about France and Belgium, and current events about the war. In addition to sewing skills, other aspects household economy to be taught included clothing care and repair, and clothing the baby. (See Chapter VIII)
The manual states that "The garments to be made may seem somewhat unattractive in color and design and materials used. Remember that we cannot expect the French and Belgian people to change their habits and customs and if we wish to be truly helpful we must not try to force our opinions and practices upon them when they have definite ideas as to what they wish." (p. 301) In particular, the Belgians were thought to have a preference for dark colors, though part of this may have been due to limited resources for laundering.
At this time, the term "morning blouse" appears to be used for a garment worn at home while attending to the morning's household chores. In the February 15 issue of Vogue magazine for 1917, patterns for morning clothes and sports clothes are shown on page 82. (A little confusingly, the model wearing Vogue's stylish version of a morning blouse is shown holding a tennis racquet.) The construction and materials used would allow the morning blouse to be laundered at home.
By the third year of high-school, students could make the morning blouse in "flannel, outing flannel, or very heavy galatea, dark colors only." (p. 362) At this time, flannel would have been understood to be wool flannel, while outing flannel was made of cotton. Galatea was a firmly woven cotton fabric, typically twill or sateen weave, usually used for nurses' uniforms and children's clothes.
Note that the instructions on the back of the envelope explain how to make a flat felled seam, advising the maker to observe how the sleeves of a man's shirt are sewed into the armhole. This tells us that such seams were common in men's shirts but probably not in ladies' clothing. The strengthening provided by a flat-felled seam justifies the additional time it would have taken to make the seam.
Here is a front view of the blouse made up in a dark cotton remnant, both without and with the belt: