They use images from a previous day to calibrate current images, as a sort of pseudo-dark frame in order to eliminate the sun's glare and allow the heliosphere to be seen. If there is a planet in the previous image that they use, it will create a "ghost" of a dark spot in future images from where the planet was. In this case, it's Mercury, from the first half of the day on August 21st. Those frames formed the "pseudo-dark" that caused the ghost you're calling "the dark planet."
[link to img11.imageshack.us]