That's a fun question! It's hard to say definitively which planet is the "blandest" as beauty is subjective and different planets have different features that might appeal to different people.

However, some contenders for the title of "blandest" based on visual appearance from space probes include:

* Mercury: Mercury is a small, cratered world with very little atmosphere. It appears gray and rocky, with no visible signs of weather or geological activity.

* Venus: While Venus is shrouded in a thick, opaque atmosphere, the surface is actually quite varied and interesting. However, the thick clouds make it appear featureless from space.

* Mars: Mars is often considered a "red planet" but its color can vary greatly depending on the time of year and location. It is also a very dusty planet, which can make it appear quite dull.

Ultimately, the answer to which planet is the "blandest" is up to personal preference. But it's safe to say that Mercury is a strong contender, as its lack of atmosphere, color variation, and geological activity make it a rather unassuming world from a visual standpoint.