Bladers are great tankers due to their huge amount of def and hp. Bladers may lack of dmg in beginning but later on they are able to use strong blades which allows them to do great damages. This mean at high lvl (60+) bladers need to rely on their equipments alot. Bladers are great at either solo or in parties. At lvl 60+ other characters start to need parties to lvl effectively, but bladers can solo effectively too.
At beginning bladers do lack of money, unless you sell all the drops/crystals to higher lvl players or npc them. Otherwise it is hard to keep on with pots and decent equipments. Some of the quests will be hard for bladers, thats why they usually need a healer to aid them on quests.
The most disadvantages for bladers are lack of attack speed or attack dmg/rating.

About Bladers build, there are alot of them but I'll show the one Im using and is most popular blader build in VN version:

*Note* All skills are passives, you cant raise the active skills.
From lvl 1-35 you will have 34 skill points to add.
Max the first skill [max lvl 20] - the first skill raises your min damage, as bladers lack of dmg, they need this skill to raise their dmg.
Add 14 points into third skill [lvl 14] - third skill raises your attack speed, as bladers are fastest attackers after swordmen, this skill will raise their attack speed to almous same as swordmen.

From lvl 35-60 you will have 25 skill points to add.
Max the seventh skill [max lvl 20] - the seventh skill your chance to do criticals.
Add 5 points to sixth skill [lvl 5] - sixth skill reflects the dmg you get from enemies back to enemies.
You can add more to sixth skill after lvl 60 if you want.