In my experience, it's pretty hard to gain muscle without gaining SOME fat unless you go REALLY slow. I started seriously weight training about 2.5 years ago at a weight of ~145 (5'11"). Two years later I'd gained 15 pounds or so, most of which I believe was muscle. Most of this gain was while I was eating a non-primal diet and I think it can just be attributed to the fact that I was new to weight training.
After being at a plateau for a while, I decided to try the Leangains approach - but with simply higher calories from fat post-workout instead of carbs (I was still pretty carb-phobic at the time). I did this by drinking a good bit of heavy cream in addition to fairly large meals. I gained about 8 pounds (to 165), but definitely more fat than I would have liked. So I cut back on calories and went back down to what I consider my maintenance weight (~158).
Next I tried eating lots of carbs, but low fat post-workout as Martin of Leangains actually recommends. I've been doing this for the past 2-3 months. I've only gained 3-4 pounds so far, but I think it's more muscle than last time. If I get up the courage, I may post some progress pics to compare my fat vs. carb surplus approaches. When I track my food, I usually have around 1/3 calories each from fat protein and carb on workout days (this is because my dinner after the post-workout meal is somewhat higher in fat).
My point in this long-winded post is that carbs are not necessarily evil for everyone. Grains, yes, but not primal carbs (sweet potato, winter squash, etc.). I think this is especially true for people with healthy metabolisms (non-diabetic and non-insulin resistant). And I guess my other point is that if you are already quite lean, it may be difficult and take a looonng time to add muscle mass without a small amount of fat gain, regardless of your diet (or maybe I'm just doing it wrong; I'm definitely open to suggestions).