Don’t know why no one answered this but I hope you found your answer by now. All mufflers have a condensation drain hole. If not they’d accumulate water inside from condensation and rust out quickly. The water drip is normal while the exhaust is heating up and drips more when cold and humid. The black you are seeing is carbon exhaust particles flushing out and sticking. You can just wipe that off or spray it with degreaser and hose it off.