First Golden Age appearance:
Mystic Comics #4 (July or August 1940):
While the outside cover has the "August" date printed on it, the inside front cover lists this as "July 1940".
As for the Black Widow herself, . . .
to be continued . . .
She would go on to have appearances in Mystic Comics #5 and #7, as well as in USA Comics #5 and All Select Comics #1. Eventually, she'd be included in the series The Twelve.