Looking at the spell progression for a magus if it had 7th level spells level 19 is where it would appear, just like if a wizard went into epic levels it would eventually get a 10th level spell slot even though it is not on the printed list. In the case of the wizard this tenth level can be reserved for say a quickened disintegrate, or anything that adds up to a tenth level spell. Is it the same for the magus? At 19 level does a magus get a 7th level slot but just lacks any actual spells to put in that slot thus he has to fill it with a metamagic spell like a wizard would. And if that were the case could he use magus arcana, spell blending, (I know they don't get one at this level but just amuse me for the moment) to get a 7th level wizard spell and then add it to his spell book as a magus spell, effectively obtaining a 7th level magus spell?
No, the Magus does not get a 7th level slot at level 19. If you look at the Wizard's table, you will see he would get his 10th level slot at 19 instead of post-20 if you just followed progression like this. The character advancement works exactly like the table says it does, capping at level 6 spells for the magus.
For advancement past level 20, there are some guidelines here. They deal with how higher level slots are allocated. But be aware, it can't be emphasized enough that they are just guidelines. Post-level 20 play isn't something the game is really set up for.
However, if you do go past level 20, and decide to go with the guidelines presented, I don't see any reason you couldn't grab a level 7 spell from the Wizard list.