What is the BFL routine like? If you're only worried about muscle mass and not so much strength, go with a BB "pumping" type program where you work each bodypart once per week using several different exercises. For instance, on chest day do bench, dumbbell bench, incline bench, flyes, etc. If you're lifting pretty heavy you might look into the squats and milk program or "Super Squats" if you're into insanely difficult workouts.
The main thing you need to do though is eat more food. In order to build any muscle you have to take in lots of extra calories. I know BFL is has you eating like 6 times per day which is good, but you need to make sure you're getting a bunch of calories. If you check out
www.bodybuilding.com you'll find a TON of nutrition info like how many calories you should be taking, how much protein/carbs/fat you should have, etc., not to mention a ton of training articles.
One program that's supposed to be amazin (I'll probably start it sometime this summer) is called "Westside for Skinny Bastards" and is based on the training principles of Westside Barbell, which is considered by many to have the best powerlifting training out there. Here's the link:
http://www.defrancostraining.com/articles/archive/articles_westside.htm