haha..alright alright...let me try one more 
time...then I give...the thing with the black 
sword, they do that in Slayers with Gourry's 
sword of light as well.  I think in that case it 
is because they digitally add the effects after 
the sequence has been shot with cels.  

I think the black paint is still for backlighting 
though ^_~  Just because if it was for sketch 
purposes then wouldn't it be on a whole lot more 
cels?  Ah but I am very happy to hear what 
everyone else thinks :D