No, but it's the best blade you can get, at least without sacrificing damage. There's a blade called "Broken Blade of Legends". I'm not sure if it's a legendary blade, because you don't get it in the Challenge dungeon, but, compared to Crimson Blade, you sacrifice 8 Attack to get 17 Defense, 15 M. Attack and 1 M. Defense.... You know what I'm going to equip Broken Blade of Legends, suddenly it doesn't sound so bad to lose 8 Attack. I actually had to type this out to see how good it is, thanks for making that question!
But to directly answer your question "Crimson Blade" is the best Blade you'll get from the Challenge floor, since there are no legendary blades as loot there. At least for now.
It's also good to use in the floor since it reduces fire damage taken.