There's one obvious way in which the legacy of marijuana prohibition will continue to negatively affect the black community disproportionately more than whites.

Although the two demographics use marijuana at similar rates, blacks are more frequently charged with possession than whites (at 2.5x the rate in Toronto, according to one recent study of first-time offenders detained for possession) .. and so blacks are therefore more likely to have received criminal records for the same. 

Since Trudeau et al have no plans to expunge convictions for nonviolent marijuana possession once legalization is passed into law, the unequal and racist application of marijuana prohibition laws will continue in the form of racist and unequal permanent criminal records. 

(It should be noted that the article was updated with much more information before I began writing this comment and before most of the comments here were already posted.)