The idea that Blackmoor's soil is infused with magic goes back to the DA modules and even the FFC/Original campaign.

The ZGG Blackmoor line introduced the idea that the magic of the Blackmoor region was so powerful that it would directly affect individual spellcasters.

This gave rise to concepts like Crystal Magic, the Wizards Cabal and the conflict between Cabal Wizards and Sorcerers.

While the Blackmoor Hardcover CS goes into detail on Crystal Magic, I have not found much detailed information about what happens if you choose not to become educated by the Cabal.

1. What exact problems do rogue wizards and Sorcerers face? We know that there are examples of spells going awry, but how does that work exactly? The fact that this was never truly defined helps add to the impression that the Cabal is only about controlling spellcasters.

2. The increased magical power seems to be limited to the North. Thonian Wizards do not practise Cabal Magic. Does that mean that Sorcerers are able to function normally outside of that region? If so, how does leaving the North affect Cabal Wizards? Do they suddenly have to begin using spell books and material components?

3. Elves are seemlingly unaffected by these effects and do apparently not take part in the Mage Wars. Are elves immune to the positive and negative effects of the magic of the North?

-Havard

I take the effect to be largely one of increasing the power of the magic (area of effect, damage, duration and so on). As I recall, DoCBm also has some specific effects listed