There is nothing wrong with apologizing for mistakes of the past. (It would be nice if the mistakes were never done in the first place, or if the person didn't wait until after the problems were revealed but instead was proactive.)
I'm white myself, but I can understand the complaints a minority might have over the issue.
Not all 'racism' has to involve neo-nazi flags and public lynching to be considered bad. Even the use of certain words or actions (like using the n-word, telling ethnic jokes or dressing in black face) could be considered racist. For example, you might not be insulted by Trump's use of the label 'Pocohantis', but to native Americans familiar with the history behind it, its quite derogatory.
Blackface is considered 'bad' because of its association with old minstrel shows back in the 1800s/1900s, when white actors would use blackface to play black characters, and portray them in a very negative way. Even if Trudeau wasn't attempting to do exactly the same thing, the association is still there.
https://www.cnn.com/2019/09/19/world/brownface-blackface-yellowface-trnd/index.html