Specifically, if a man were to marry a woman with the "mark of Cain," back in the old days, what would have happened to him?

No they have not.

So what did they do if someone wanted to have such a marriage back in the 1960s?

Well, Brigham Young did say it was "death on the spot" for those who mingled their seed with black people.

And for all we know, and knowing what we know about BY, who is to say that didnt' happen once.

It certainly wouldn't have happened twice.

so with all this mixed race stuff...what about white and asian?

that was part- Asian and part very blonde Yankee.

They had the most adorable children, were very active, and well-liked by everyone.

How many Blacks were there in the church before 1978? I think I heard there were only a few thousand. Mixed race marriage was illegal in Utah until perhaps 1967. That would have discouraged the few Black Mormons from even trying it.

I'm not sure why the poster above is so certain there were none. Since excommunication is often a very local matter, we'd have no way to know. It isn't like the church is publishing numbers. In fact, they are quite adept at whitewashing their unpleasant bits.

http://connellodonovan.com/black_white_marriage.html

Let's not forget: Afro-Americans came west with the original pioneers....
anyone who thinks there weren't opportunites for blacks & whites to be together... a bit naive there.

It was illegal for Blacks and Whites to intermarry in Utah until the 1960s.

From "On This Day in Mormon History:"

"Feb 8, 1844 - Joseph Smith's only acknowledged polygamous child Josephine is born. Her mother Sylvia Sessions Lyon is legally married to Windsor P. Lyon with whom she is living, and so this is the first acknowledged polyandrous child.
As chief justice of Nauvoo, Smith fines two African-Americans $25 and $5 for "trying to marry white women."