Blackwater will be fine regardless of snow depth. I've been up there w 2-3 ft of snow depth and while the roads can be snow packed, they are drivable.
There will be no driving access to dolly sods. The FS roads that provide direct access are closed in the winter. You may want to look into going to whitegrass ski area and paying their small day use fee for hiking and snowshoeing. You can hike up above 4000ft and when you're done, get a hot meal at their hut at the base.