http://elderscrolls.wikia.com/wiki/Perks
Says ;
Bladesman: (3) Attacks with swords have a 10% chance of doing critical damage (+5% per additional rank)
It's a bit unclear on it's own but when you look at the first perk in that constilation;
Armsman: (5) One-Handed weapons do 20% more damage (+20% per additional rank)
It would seem that daggers are not included in Bladesman but that's just my opinion based on the wording and obviously they will be included in the Armsman perk which you can take 5 ranks in so that's something. That combined with the possible 15x back-stab multiplier means some serious damage (especially with added poison) if that's your play-style.