Jack Johnson (beginning in 1908) was the first heavyweight boxer to fight (and defeat) both Euro and African American opponents to achieve the crown. All the persons holding the title before Johnson (and many after) refused to fight African Americans, by all indication because they believed they would be beaten. They most likely would have.
To put it crudely, Jack Johnson was the first Heavyweight Champion of the World who didn’t deserve an asterix beside the title explaining how he was too chicken to fight his most dangerous opponents.
In my book, that makes him the first champion. Period.