How can I print out a PDF substituting pixels for blue pixels?

Install Ghostscript (first) and then ImageMagick, and then you can use the following command:

convert -density 300 input.pdf -fill blue -opaque black output.pdf

This will convert all the black in input.pdf with blue in output.pdf.

[Thanks to Nick's comment below for the part about ghostscript.]

Note added later: If you have a newer version of imagemagick that doesn't have a convert binary, use magick convert instead of simply convert.

There's a free online tool that's built for this very purpose:

https://supertool.org/automatically-add-color-to-pdfs-to-print-without-black-ink/

It took a little while to load and process, but it made it possible to print a return shipping label without black ink.

Maybe you can set an option in the printer driver to print black as composite from yellow, magenta and cyan.