I was thinking she was some type of a Bombay/Burmese mix myself - she's a very small (slim) cat. And yes, she does have a small tuft of white below her neck. In addition, she also has a large patch of white fur on her underside.
Apart from that, although she looks pure black otherwise, I've noticed she has what looks to me like a striped tabby pattern underneath her fur, if that makes sense. Under the right lighting (like when using a strong camera flash), it's quite apparent. I'll have to try and get a pic of that.