Edit:
Since yesterday I’ve been tinkering with the api and I’ve just found a lead
The * argument doesn’t work, I have to manually add all the image names for it to work
?populate[blocks][populate]=*
?populate[blocks][populate]=carousel,background,badge,thumbnail,*
The “background” image and “badge” work with * but not the others
this is quite a surprising problem
I hope my research will help other users
Have a nice day