Juan,
As I understand it on the 747 the number of trip lines plus seeded blind lines plus reserve lines equals the effective numbers of bidders. So if you bid for everything you would get a trip line or seeded blind line unless you were allocated reseve.
As Hotel Mode says you will only get a blank blind line if you did not bid or you did not bid for enough i.e. everything you bid for was taken by someone more senior.
Regards