I work quite extensively on several different Wikipedias, and CAPTCHAs are normally only a minor nuisance for me. I was importing templates to the Zulu Wikipedia when I was met by the CAPTCHA code BAUERNEGRO. What is this? Are we not applying any filtering at all on the words we use? This was after already being prompted to write gipsydick. This is extremely offensive, and not acceptable from a major website like Wikipedia or from MediaWiki.
Topic on Talk:CAPTCHA
Bauernegro - What system are we using!?
There is a blacklist but very rough.
Any way to expand it to include words such as negro, gipsy and dick?
Sure, you can send patches: for instance I have one which does that among other things, gerrit:121255 (based on Wiktionary).
However it may take months or years for the new blacklist to take effect, as it's only used when the captcha images are regenerated. We also don't have control on what sort of dictionary WMF uses.