Please note: this is meant to be a general, digestible summary; an exhaustive report could be a book unto itself.
The iconic "blue jeans" that somehow endure but are also constantly new have such a strong association with the present that I find a lot of reenactors and living history people downright phobic about implementing trouser fabric that even vaguely resembles what is currently known as "blue jean." Not helped, surely by the horror stories most have us have heard and witnessed of some folks blatantly or mistakenly subtly utilizing modern blue jeans in pre-1960s contexts.
For a corroborating perspective/further reading, Ms. Downey has an excellent piece here geared toward history and myth busting regarding Levis here:
A wide a range of shades of blue have been utilized in blue jeans and referred to as "denim," including cotton weights and weaves, which do not resemble at all the heavy cotton drill to which the name was originally applied. The result being that nearly any blue dyed cotton fabric made into trousers makes many reenactors cringe at the inevitability of some ignoramus (fancying himself a clever fellow) inquiring about their "blue jeans."
But work trousers, cotton and otherwise, of such a color and even texture were not an innovation of Levi Strauss at all. The only distinguishing feature of his pants at the time actually seems to have been the rivets. Modern Levis, however, do not resemble denim/jean/indigo trousers of the 19th Century. Even in their classic/early cut versions, the waist line is noticeably low as far as the 19th Century is concerned. Likely because it would seem too foreign to modern customers.
Jean, itself, is a modern misnomer for the fabric of blue jeans. In the 19th Century, "jean" referred to a blended weave of wool and either cotton or linen (linen being either flax or hemp). If the cotton or linen was left natural, with the wool being dyed blue, it can resemble the modern concept of denim or "jean" from a few feet away. This is a legitimate choice for trousers and other garments even early in the 19th Century, when such goods were domestically produced rather than imported (making it frugal for them; modern recreations have often swung the other way price wise). Examples include these breeches belonging to Isaac Shelby, Kentucky's first governor, at the Kentucky Historical Society (ca. 1815):
And this frock coat at Western Kentucky University, accession # 1930.19.3, ca. 1850-60:
And here are some examples of actual mid 19th Century trousers made of blue cotton that to modern eyes might be thought of as denim (though not necessarily qualifying for the technical term).
First, some well known ca. 1840s trousers from the Museum at FIT (thanks to the Two Nerdy History Girls for providing an accessible report):
And second, some details of a fly front pair in a private collection. They are mostly faded to nearly white but some less exposed areas reveal that the original color was blue. Date is difficult to determine. They are very plain and entirely hand stitched. My impression is no earlier than the 1850s and absolutely no later than the 1880s:
A survey of 19th Century art work reveals frequent occurrences of such trousers (specific textile, of course, being impossible to determine). Just a few examples:
Leisure Hours by William Sydney Mount, 1834
James Goodwyn Clooney, Mexican News, 1847
James Goodwyn Clooney, 1847
Rail Shooting by Thomas Eakins, 1876
Even more than two decades after Levi's famous patent, such clothing, in denim, specifically, was still just commonplace work wear. Page 178 of the 1897 Sears, Roebuck, and Co. catalog advertises several "denim overalls" corresponding to images that are what would now be thought of as trousers with "apron overalls" (more like the modern concept of the form of overalls) being a separate product. None of the items were produced by Levi Strauss Co.
NONE of this is intended as a slight to the Levi Strauss products; just as a clarification on their context in the broader 19th Century world.
Yours & c.,
The Victorian Man