Coffee has been brewed using many different methods for hundreds of years, you can read a brief history here. Initially coffee was brewed Turkish style with finely ground coffee mixed in with the boiling water. In the 19th century it was made by boiling the coffee grounds in a cloth bag, which had to be cleaned each day. Modern filter, or drip coffee didn't take off until the early 20th century.
The Mother of Invention
It was a German Housewife called Melitta Bentz that invented the first paper filter coffee system. In the early 1900's she grew tired of the various coffee brewing methods that always left grounds in the cup and started experimenting.
She ended up using a punctured brass pot with blotting paper covering the holes. This produced a full flavoured clean cup that was met with great enthusiasm, so in 1908 she started a business, and the rest is Melitta history!
What are Bleached Coffee Filters?
Generally unbleached paper is light brown in colour, so to produce paper that is white, it has to be bleached. This can be done using chlorine, or oxygen bleaching (hydrogen peroxide).
Chlorine bleaching is effective, and doesn't leave any residual flavours, but is more harmful for the environment, so many filter manufacturers have switched over to oxygen bleaching.
Unbleached Coffee Filters
Unbleached coffee filters are brown in colour, and require less chemicals in their manufacture, so are kinder to the environment. Like many things in coffee, the arguments for bleached vs unbleached has created a lot of debate about the impact on taste.
Rinsing Away The Papery Taste
This process ensures that you don't get any residual papery taste in your coffee, and it also warms up your brewer, which will keep your coffee hot for longer.
Which type of filter is best?
If you are 100% all about the environment impact, then you have to go with unbleached. It's better for the environment, even though the bleaching process is getting more environmentally friendly all the time.
Personally, I have never been able to get a good flavour from unbleached paper filters, so I choose to use the white oxygen bleached ones. Just check that they are oxygen bleached, and not processed using chlorine, it should say TCF (Totally Chlorine Free) on the packaging.
If you want to test the performance yourself, just pour hot water through both, let it cool, and taste it. Sounds disgusting I know, but it will tell you exactly what flavours you are imparting into your cup that don't come from the coffee.
One last thing
If you want to be really fancy, you could try using the Hario Bird Filters! No change to the taste/performance, but they look funky!