One such episode was Math's notes about meeting black people from Minneapolis in Berlin, in 1914. They were performers at the Wintergarten. Part Six of the diary says "Met 2 coon's. They are from Minneapolis. One sings here in a show house, the biggest in Berlin. Have heard him sing cause I have been to the show".
So, we researched what had to be a very small subset of singers (black) from the US (Minneapolis) who played Berlin (the Wintergarten) in 1914, and we found a likely couple called Johnson and Dean. (...An act with two people, neither of whom was known as a good singer, so it's probable Math's 'singer' was someone entirely different). Here's their resume for those years from a book called Notable Black American Women, on Google books:
We also found that Johnson and Dean broke up in the summer of 1914, and each went their own way for the next 20 years. But in Berlin, in May 1914, Math could very well have talked with Mr Johnson and another man hired to be in the act (see below). (Ahem...knowing Uncle Math, if he'd met Dora Dean, he would have mentioned it!)
In another place in the diary, Math says there were four black performers...hmm. Possibly two different acts? But then, our Master Researcher found this item in Minneapolis' African American newspaper, the Appeal ►.
And look, we hadn't found any mention of a son before, but checking Family Search for a Herman Johnson who was NOT from Norway or Sweden, we found:
Anyway, none of this proves they were on the Olympic with Math and Theo Hesch in April 1914, or that they met in Berlin. After all, they were from provincial Buckman, Minnesota...would Math have approached an unknown black man? We think so, because as Larry pointed out, Math and Theo would have known Schmolke's chauffeur Wilbur Nevils, who was listed on the 1910 Federal census for Buckman. I suspect Math felt pretty cosmopolitan...lol
Added over a year later: two mentions of Wilbur Nevils in the Pierz Journal, but look, both in July 1910. I suspect Mr Nevils wasn't all that comfortable in Buckman. Larry found him living with his wife and kids in Minneapolis in the 1920 census.