I supplied the negatives. Either you want a real and actual blog that gets search engine attention, draws organic 9unpiad) traffic, gets backlinks from other bloggers to your posts and achieves a PageRank or you don't.
If you don't care about those things then you can keep search engines out and create a private journal.
There are 3 options. See here > http://en.support.wordpress.com/settings/privacy-settings/
I would like to warn you though that if you do choose option 2
I would like to block search engines, but allow normal visitors – If you want all human visitors to be able to read your blog, but want to block web crawlers for search engines, this is the setting for you.
and any of those who do visit your blog choose to backlink to one of your posts their post will be indexed and there will be evidence that your blog exists on the internet.
It would be so much easier to answer you and provide solid specific information if you would simply state what your concerns are.