If your check valve in the pump has failed as previously mentioned as a possibility and you cap the blue line the tank will fill up over a couple days or more and begin to flow out through the tank vent at the tank fill cap and run down the side of the trailer. As long as it is just a trickle you are probably okay. If it stops trickling and continues the fill the tank then you may rupture your tank. The previous owner may have known the fresh water was leaking back through the tank and left the cap off so the tank wouldn't fill. You may need to repair or replace the pump if it is back filling the tank.