Why the authors believe anyone would be particularly interested in looking at the blue sky when they launch a 'planetarium' on a rainy day is anyone's guess. The reason you may get a blurred picture of a field with a blue sky without any star or planets when you start the Stellarium 'planetarium' is that it will intentionally try to show you exactly what the sky looks like when you launch the program. You may get greeted with something like this the first time you start what is supposed to be a 'planetarium' or 'astronomy software' on a GNU/Linux distribution: Stellarium is not for computer novices or people who aren't interested in software beyond it's utility, if you just want something that lets you look at the constellations in the night sky real quick without having to learn how some advanced planetarium program clearly aimed at professional astronomers works then KStars or even the simple web version of Stellarium would be better alternatives. You can learn how to use the GNU/Linux desktop version of Stellarium if you have an interesting in and familiarity with clunky software.

It is actually a quite advanced planetarium program with a lot of advanced functionality beneath the joke of a user interface that is filled with everything but the basic essentials you would need to get any benefit out of it. Stellarium is a pretty advanced free software planetarium program with a user-interface that gets so many basic things wrong that you will likely judge Stellarium to be a completely useless and worthless piece of software the the first time you try it.

Stellarium free software