Conventional usage of this word in the sca refers to a garment known from the 12th century (actually possibly dating back to about 1060, and forwards to about 1210 in remote areas). This garment differs from the t-tunic that preceeds and accompanies it in several ways. It appears to have been a court garment, ie used on special occasions by those who could afford special garments used on days when not doing any physical labour. The body section of the garment (hips to underarms) was much tighter than the normal t-tunic, for the most fashionable kinds of bliauts, the sides of the garment were laced up with spiral lacing, to get a tighter fit. Women wore a bliaut with moderately full ankle length or lower shirts. Men wore garments that came to just above the ankle, but had less fabric in the skirt and were slit up the side to nearly the hip to allow free movement. A wide range of sleeve types were worn with this dress, from tightly fitting sleeves, to sleeves that flare out into a dangling cascade from anywhere between the elbow and wrist. Such sleeves might be knotted if especially long. Women tended on average to have longer sleeves than the knights and princes depicted in this type of clothing. A large variety of sub-styles existed within this broad definition, depending upon country, exact time period, rank and personal preference.
You will be able to find plenty of medieval pictures of this kind of garment if you look for books under the topic romanesque art, but there are no excellent books on how to construct this kind of garment. Many costume books are based on the victorian idea of this kind of garment and are of dubious value at best, downright confusing and wrong at worst (even some which are great for other periods).
The word bliaut can be used to refer to many things:
- It can refer to this broad style of clothing, as above
- It can refer to a specific french sub-style of this clothing, as depicted in the statuary at Chartres cathedral
- In Medieval french texts the word is possibly used to describe all outer garments (tunics, dresses, not cloaks), despite whatever style they are cut in. This usage appears to last until the 14th century.