How to properly override Bootstrap variables?

Question

Sass

I'm trying to redefine the bootstrap variables, specifying what I need before importing the bootstrap itself, but I get an error that the variables were not found. For example: Naturally $black is not defined. But how then to be? If I import the bootstrap before the override, there are no changes (ignored). If you import a file with variables separately before the override, then errors appear that there are no mixins.
$body-bg: $black;