I'll try to offer some help.
1. Regardless of variety, there still lots of variation in onions, especially the open pollinated ones (non hybrid). As long as all the necks are well dried with no moisture, they should keep.
2. The black stuff is a type of mildew, i've had it more some years than others, probably due to rain or overhead watering. It never seems to cause any problems other than superficial.
3. Soft spots may be from the concrete but more likely are from bumping the onions during or after harvest or from sunscald in the field. I'd thing the best option is to eat them first as they probably won't keep all that well.

4. My onion harvest process is:
1. stop watering at first fall.
2. 50% down, kick the rest down.
3. wait a week, dig onions.
4. if weather permits, field cure for a week taking care not to sunscald onions.
5. when tops are completely dry trim to about 1" and trim roots.
6. cure in well ventilated, warm place for another 5 days (I use the greenhouse with a fan)
7. store in dark, dry, cool conditions.

I just finished curing about 1000 lbs. of overwintering and the spring planted are just starting to fall.

Hope this helps!

Thanks for your response.

I have some trays that I have made with hardware cloth on the bottom. After drying on the porch and trimming the tops and roots, I put the onions in the trays to complete the drying process and find that the soft spots seem to have taken care of themselves.
After about a week, the onions go into mesh bags and into the cellar where it is much cooler and dry.

How do you field cure and yet not let the onions get scalded by the sun?

We lift them and then lay them down with the tops from one onion laying over the bulb of the next and make a long 3ft wide windrow like this. On a small scale it works well to lay them like this in a sunny spot on a pallet or dry straw.

Glad to hear that the soft spots cleared up. Maybe the cement was the cause?

Madroneb, I'm curious about the overwintering onions. Do you fall plant them like garlic? How are what is produced any different from your spring planted onions?

I'll try to sum it up quick.
Onions can overwinter if your temps don't drop much below 15F for any long periods of time.
Seeds are planted around mid August either in the field (if you can control weeds during their slow growth), or in flats or a nursery bed and then transplanted to their permanent spot around mid Oct.
They grow for a month then slow until late winter when they kick in and usually finish up by May-June.
Some varieties are much better suited for overwintering, Walla Walla is famous in the West, but there are reds and yellows too.
The benefit for me is that they finish early so I get a good price, I can grow them without irrigation and they are in the ground already when my other busy spring plantings are happening.
The only detriment is that they generally don't keep much past November.

We can grow a garlic, couple of bunching onions, multipliers and yellow nest onions through winter, but do best with bulb onions grown from seed, and spring-planted shallots.

With practice, garlic curing becomes an art in itself.

Here is a link that might be useful: harvesting and curing garlic photos