What are the conditions for Genis to learn Stone Blast?

Different sources say the same, the only requirement for Stone Blast is to reach level 3, as you can see below.

What needs another spell to be cast specific number of times are Mid and Advanced spells.

Here you can find other useful tables with spells and their requirements.

Regarding the blue spell:

The spells in blue means that you have the prerequisites fulfilled but needs to randomly unlock it in battle. Using a prerequisite skill or an A attack (for melee techs only) will unlock the skill. The randomness and likelihood of unlocking a skill is based on the character's Luck stat.

Source, Neoseeker forum.