Both Blade & Spawn were made back in the 90s. They might have contributed a little bit to BP, but I don't think they really helped that much considering that 20 years have passed and except for the first Blade movie, these movies weren't very well received critically and financially. BP's success has a lot more to do with MCU's brand recognition imo.
Initially i think the first Blade film stayed more to the huge Mavel fan base
and B-movie like goers, but it generated legs and interest even with
the non-comic general audience. Also what could have helped it further
was the vampire factor which long has been huge among general public.
And so true that Blade wasn't in the general critics as sensational
must see and i'm willing to bet that they saw it as guilty pleasure than
a cult classic mainstay. $37 million box office opening was more than
enough to spin more Blade sequel and eventual trilogy.
Spawn featured a black human character but as a burnt-disfigure
and full costumed creature feature, i'm not sure it's recognized by many
as a black superhero film. And again as u pointed out, it's just not critically
acclaimed and nothing investing in at he box office to even think about
more Spawn films. It was clearly one and done.