Fixing bathtub faucet

My wife had been using some sort of blue liquid in the toilet tank which has left blue lines in the toilet bowl. How do I get these blue lines out. I have tried scrubbing with soap and also spraying with undiluted bleach. Nothing helps. Any help would be appreciated. Thank you. Also, I want to thank you for the information you gave me about how to open a clogged drain in my bathtub. Thanks very much. You really know your stuff.

We have been in our house for almost 8 years and just a few weeks ago have started having a very foul smell coming out from under the counter in our master bathroom. It is definitely a sewer smell and is VERY strong. It is not constant, however, and seems to get worse when it is cold outside (though the sink is not on an outside wall). The bathroom is close to our laundry room and we had one plumber tell us he thought the problem was the washer pulling strongly on the water line and getting all the air out of the bathroom sink trap. He told us to run water down that sink drain when we use the washing machine. That hasn't helped and there doesn't seem to be a strong relationship to when the smell is bad and when we've run the washer. Any help we can get before I call another plumber and waste more money would be greatly appreciated!!!