You can use `perl`:
echo -e "aaaaaaaaa\
#some info\
other rows\
end row#\
another info" |\
perl -0777 -ne '/#([^#]*)#/ && print $1,"\
"; '
Explanation:
* `-0777` slurp the whole file as one line (enables multiline matching)
* `/#([^#]*)#/` match non-# characters `[^#]` between too `#` and with the brackets add it as first matching group.
* `&& print $1,"\
"` if found, print first matching group and a final newline.