i called it a bug but it might be by design. plus it's too well-known, as written in guliverkli in 2004 "\fad-fades tend to blur into gray instead
of becoming transparent smoothly."
http://sourceforge.net/tracker/index...03&atid=565649
He or she said "gray" probably because his/her border color is black. A better solution than \t would be to use layered \fad fades (much less cpu-intensive w the same results), but I was too lazy to post it there. I doubt there's any typesetter who doesn't notice it.