OK, but... does this mean you don't watch anything on Youtube?
Figure out what you want, and then put together pieces that get you there.
Basically you'd use the "Get Current Webpage from Safari" block, then in the Terminal
PATH="/usr/local/bin:${PATH}"
cd Music/YTDownloads
# or wherever you want it
youtube-dl -q -f 140 "${@%%&*}"
# format 140 is 128k .m4a
Then add to existing playlist in music. Ah, if only it were legal!