I don't think there was or is a "law" against blackface. Now they just label it a HATE crime and prosecute for that. Hurting someones feelings.....The "snowflake catchall law"...!!

College won’t punish students for blackface photo, citing First Amendment September 12, 2019 By Joshua Rhett Miller

Four college students in Colorado posted a photo of themselves in blackface, setting off outrage online — but they won’t be disciplined because they’re protected by the First Amendment, university officials said.

The photo, which was posted on Instagram over the weekend, shows four unidentified white Colorado State University students in blackface with a caption reading “Wakanda forevaa,” a reference to Marvel’s “Black Panther” hit film.

The image was quickly blasted by critics who demanded that university officials get involved.

“Anybody that tries to justify this behavior is equally as ignorant and offensive,” one woman wrote on Facebook.

One man who said he attended the university posted on Facebook that the image was “not surprising” at all, while another detractor characterized it as “disgusting and absolutely unacceptable” as they called for consequences, a Facebook thread shows.

But university officials insist their hands are tied, saying that personal social media accounts aren’t under its jurisdiction.

“Our community members — students, faculty and staff — can generally post whatever they wish to post on their personal online accounts in accordance with their First Amendment rights,” CSU president Joyce McConnell wrote in a statement released Tuesday. “This recent post runs counter to our principles of community, but it does not violate any CSU rule or regulation, and the First Amendment prohibits the university from taking any punitive action against those in the photo.”

BUT...

G***g G********w, former Works at Lawyering Now and Again

If an act was legal when committed, one cannot be prosecuted for it simply because the law subsequently changed. Having read other answers I will qualify my answer to say my answer is limited to Canada, USA, the UK and other legal systems based on the British model. Other systems outside of this set may well do ex post facto prosecution
OR

Usually no. That’s called Ex ponst facto law and there’s a whole Wikipedia page about it.