I think blended learning at primary could have turned into a nightmare, to be honest, if not done carefully.
A lot of families just wouldn't have childcare (including teachers themselves, who generally rely on mostly being off at the same time as their kids, so don't have alternatives in place), so kids would be dropped off at each other's houses or left to roam around. I know several parents in DS' class (including myself) who would have to quit work or massively reduce their hours if the kids were at home 3 days a week - the furlough scheme wouldn't cover that, so it would just mean a fairly substantial drop in family income.
Teachers would be preparing online lessons but also still teaching full time, so a lot of work.
DH is a teacher and I think he had about 3 kids out of 22 or so who actually did ANY of the online work he set during lockdown - between families with no access to tech (or one computer between 3 kids, or broadband that wasn't up to parents doing conference calls and kids online as well), ones who couldn't be bothered (or parents couldn't be bothered setting it up for them), and ones who just vanished off the radar completely.
I think in secondary it would be more successful, because at least the kids are old enough to stay at home alone.