Now that I have upgraded to a newer version, I don't have that option anymore either. I'm sorry, but I can't find where that blue line is configured. I thought it might be in the Reply template but I don't see it there. Hopefully someone else will know the answer.
I volunteer as a moderator to help keep the forum tidy. I do not work for Ritlabs SRL.
My suggestion is to use TB!'s feature of alternating text color for quoted content. I'm assuming the intent is to distinguish quoted text from responder's text. And the feature of setting color for odd and even text does an admirable job of that - and leaving the background white ensure the recipient will be able to read the text easily. Good luck, david