Many of my terracotta pots have turned a very unusual color. They are black, brown, mottled looking. I have the Chinese asparagus plant in the
The mottling that you are seeing could be a number of things but it is probably unrelated to the asparagus plant.
Terracotta pots may be made with many types of additives and impurities that can leach out of the pot over time and look unattractive. The plus of terracotta is its porosity but that can also be a drawback if you don't get very high quality pots. There is no regulation as to what can be in the clay so it really could be anything. That is the most likely source of your staining issue.
The second possibility is that some element of the plant food that you are using is staining the pots. You are probably familiar with the common white salts that accumulate on the exterior surface of used terracotta pots, but there are feeds with ingredients that could make a darker or greasy, stain.
Finally, you may have a mildew, mold or fungus problem but that would probably have an alarming texture as well which you do not mention.
I hope that helps you with your problem.
Courtesy of NYBG Plant Information Service