First, I've got to admit that i don't have a 3D printer, but I have a friend who prints stuff for me.

For this project, we need ABS as material. My friend has several years of commercial experience and research in this area, so it take this as granted.

While he has a good match for "Light Bluish Gray" and "Dark Bluish Gray", his "Blue" is off in comparison to blue LEGO bricks.

Has anyone found a close match that would be worth trying?

I don't have a filament recommendation for you but perhaps you can use https://filamentcolors.xyz/colormatch/ to find a potential candidate.

For what it's worth the "Lego Blue" is 0055BF

Which gives several plausible PLAs and a PETG, but no ABS sadly.

Ah ha! We have a winner.

https://filamentfinder.thinkonezero.com/?c=0055bf

Using RGB values for physical colors in not ideal…

I've used this for making wall prints that don't stand out against the paint. It's quite handy

Looks like the filters get defaulted to pla and petg, and a lot of vendors dont make the default cut when following the link, but there are several matching options for abs and asa.

Yeah couldn't figure out how to add the filters to the URL. But yeah there are some promising ABS/ASA options!