when ever i build a black fire my cdr is always lacking 5%

is blackfire supposed to be 15% but gives wrong amount or is it supposed to be 10% and just labeled wrong ?

I've had UI display issues with plenty of CDR items before, mostly just with it not showing up though. Moving the item in your inventory generally solves that one.

If it's displaying a different number than the tooltip states it's probably getting the stats tweaked around without the UI portions being updated to match. Which I've also seen a few times, mostly in the PBE were item attributes are a little more fluid. If this is the case you should probably post it in the Bug Reports forum.