If your nettles are this late in the game, don't pick them for the leaves. You want nettles earlier in the year, before flowering. And, if you keep cutting nettles, you can get several harvests before they set seed. David Winston, a respected herbalist, uses nettle seed for some kidney disorders.

Do you think the ones I just cut would be ok as hay for sheep, even though they have blossomed? Normally, they relish dried nettles.

Also, I cut these back to about 1 foot high stems. There are no leaves on them, but I expect they will re-sprout. Would their leaves be safe even though they previously blossomed?

Nettles cut this late would still be good in the compost pile, while the re-growth will be tender and fresh like spring growth- BUT not for as long a time. I'd let the leaves get full size and cut them.