CBD-1425:  change how BLANKFILE is referenced

Change-Id: Ic950e82a143ad55fd4d7cf932e08be6e44b8deb4
Reviewed-on: http://review.couchbase.org/40636
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>