There's one apparent rule that elites among the Liberals, Conservative and NDP all seem to practice in their apparent discretion.  It's potentially okay for black people to represent the party in ridings where there is a lot of black people.  However, if you're black and you're riding is mostly white with few black voters, party elites will seek to block you even though as a black person you might have earned your nomination fair and square with the support of mostly white members in your riding association.

I have had personal experience with this practice is all three major political parties and I have seen not only other black people the targets of its practice but also other so-called "visible minorities". Indeed, one time I went into Ontario Liberal headquarters to protest this apparent practice and I met someone of South Asian background who was also the targets of its practice.