I think it is a part of "それ(/あれ/これ)くらいのこと", which means "such a small/easy thing". If it means "dark stuff", it should be "kurai koto".

I think it would be better to get some context - that would make it easier to know which 'kurai' is being referred to. Personally, the only 'kurai' I can think of is 暗い - meaning 'dark'. I would roughly translate 'kurai no koto' as 'Dark stuff'. But I'm not sure whether that's the proper meaning. Aside: If, however, the phrase is actually 'kirai no koto', that would mean 'Things I hate'. Anyway, I would suggest googling the song lyrics (assuming you know its title). Good luck.

ok http://www.youtube.com/watch?v=d87fYuIX6Gw here it is :)

By the way, can you write the name of the song? I would want to listen to it too since I am in love with Japanese songs too ^_^