A JSON map consists of the following:
{
"mapOptions": {},
"operationalLayers": [],
"baseMap": [],
"exportOptions": {},
"layoutOptions": {}
}
import json
from arcpy import mapping
mapService = {}
jsonDump = json.dumps(mapService)
result = mapping.ConvertWebMapToMapDocument(jsonDump)
mxd = result.mapDocument
mxd.saveACopy(r"c:\temp\blank.mxd")
Now you can do whatever you need to do with your map document.