I believe that Disney wouldn't release Black Widow on Disney plus for now, according to Victoria Alonso, Marvel movies are meant for theatrical release, but when 29th April comes, and theaters are still close, they have no option but to release it on Disney plus and VOD

If it gets to the point that a whole other year goes by and we can't watch it in theatres, then yeah, of course they'll put it on streaming. But, I can almost say for certain that they'll keep it to a theatrical release (as they haven't done it already, so I doubt they'll change that just because DC did it... Although...)