Generally speaking, I would like to know the optimum or best % split of visitor sources (like Organic Search, Direct, Referring Sites).
All depend upon your blog/site. If you have newly settled blog, then you might get most of traffic from Google - Something around 60% and 30% referral traffic, such as twitter,facebook or any other sites. and some direct(This may be of yours only) If you have old blog has tons of subscribers and its popular too then more likely your visitor just enter the site url. But still majority of your traffic will come from search engine. For my site its about 70% from search engines and 10% direct.
The more referring and direct traffic the more you will be on the safe side of receiving traffic and returning visitors. the last Google updates and changes have proved to even top sites, it's not really recommended to rely on organic traffic only. Google or other search engines can send you good traffic but in the blink of an eye this stream of traffic can be shut off due to any changes the Search engines might make it their algorithm. If your sites has some good backlinks from other trusted sites, you will see a good and quality stream of traffic.
It doesn't really matter as long as your traffic is targeted enough. People more prefer SE traffic just because SE traffic known as targeted traffic.