How can I export contacts and tags, but only for a particular tag?
Yes, you can export tags for contacts with a particular tag. It doesn't seem you can export just one particular tag, but you could easily do this as a post-process once you've exported to CSV.

Checked on http://dmaster.demo.civicrm.org/ -


Search => Advanced Search
"Basic Criteria" group => Tags => Volunteer
Submit "Search"
Click "All 61 Records"
Select "Export records" action
Export PRIMARY fields
Exported CSV contains tag information ...