I believe that using a crocheted granny-square blanket in the 1795 scenes may have been anachronistic... Until the 1800s if I'm not mistaken, crochet mostly existed as a means of reparing knitted items, catching lost stiches, etc. Crochet as an independent entity apparently orginated as a variation of tambour embroidery nearer to 1800, but not yet so developed as to make large items. Irish lace crochet with fine threads was popularized in the mid-1800s. Nuns often taught it to poor girls as a way to imitate fancy woven laces and thus earn them some kind of living, and it caught on from there.
The original "afghan stitch" according to my numerous needlework books, is more a combination of crochet, knitting, and weaving, making a fabric rather than filet netting or open-laced motifs, of which granny squares are obviously a variation. Using chunky yarn remnants in the granny pattern, not sure when that came about--- more common in the later part of the 19th century, though. In any case, a rich variety of colors wouldn't have been available in those days. (But then again, probably neither were the blue bed sheets DS used!)
The kind of blankets most popular in the 18th century were woven, or traditional patterned patchwork quilts, or "candlewicked" quilts, where wicking was worked into a raised pattern on a monchromatic background and then tufted. Inexpensive machine-made versions of the latter were still being sold in the 1960s--- my parents had one on their bed, I remember. Maybe Ohrbach's didn't sell those? Still, how nice someone used the afghans, wherever they came from, and "immortalized" them. Even a simple afghan of bedspread size could take MONTHS to complete.
