I was wondering what could cause a world wide blackout of electricity that would last a few hundred years, as well as reduce the human population to 30% of what it is now?

Some scientists believe a large enough solar flare could knock out the power grids on a global scale which would leave the earth in darkness for around a decade or more.

Soul Fly gives a good example of a natural disaster.

A "man made" cause would be widespread use of EMP.  A few devices set off in orbit would have the same effect.

And without electricity, I don't doubt there would be widespread death in developed/industrialized nations. 

Just the lack of air conditioning in the summer, or heat in the winter, kills thousands of people in those nations each year.

No electricity = no refrigeration = food spoiling before it gets to market = starvation on a massive scale in large cities.

Not to mention the lack of medical care available, without electricity.

We'd basically have to start from the level of steam engine technology, and rebuild to our modern level.