You can blink during a long exposure and not have it show. Those really old pics could take as long as 2 or 3 minutes. So a blink would not be an issue, but any movement of the head or facial muscles would result in a blur.
I have a large collection of family tintypes and daguerreotypes and am extremely fortunate to have names attached to most of them, and some stories as well. One in particular was taken in about 1870, and has my great great grandmother seated, with a child in her lap. Its rare to see daguerreotypes of very young children because you just can't keep a child absolutely still long enough. Well, my great great grandmother was a determined woman, and was proud of her children, and apparently was set on having a picture of that baby. Some baby pictures were achieved by setting the child on a fabric drape and having the parent grip the child firmly thru the drape, so that the steadying hands did not show, but my ancestress opted to simply hold that baby's head in one hand, and his body with the other. Her firm grip is the first thing most people comment on when they see that picture...but it worked. Unfortunately, if we have the dates correct, it is the only picture of that baby as he passed away, as so many children did those days, before he was even old enough to walk.
I think I have that pic saved on a thumb drive...if I can find it I will add it to this thread.