You'll need to be more specific about "footer", there are different footers (page, tablet, document, ...). I'd recommend checking the official manual https://asciidoctor.org/docs/user-manual/ which has almost all options.
About making the numbers black, deppends on the backend used.
If you are using HTML, you'll need to define a stylesheet, you can start copying the default one and modifying it. But see that the section numbers and section titles and encased in the same H* tag, so you'll be changing the color of both.
PS: forgot to welcome you on the first post ^^...welcome to the forums!