Hi Laura:
The message "Blocked (Field Management)" indicates that these fields have been blocked from being overwritten.
Go to Admin - Field Management and locate those fields. Check what their properties say about blocking.
The question is whether you'd ever want to overwrite these fields with data from imported files. Marketo should accept the import, but not update these fields on exisiting records, while importing First and Last Name for new records.
Hope this helps.