What is the purpose of "gibberish" comments posted to my blog?

They're probing your site. First, whether the comment will be published. Second, note how they use several popular syntaxes for links - it's an attempt to check which of them will result in an actual HTML link. If your site lets those posts through, expect more spam, this time more malicious.

Many spam filters use Bayesian analysis to determine what is spam and what isn't. These work by comparing inbound content with "known good" and/or "known bad" examples and looking for similarities. By slowly increasing the amount of junk in the "good" pile, an attacker can lower the effectiveness of the filter.

They are trying to confuse any automatic spam filters you might be using.

Random strings are unlikely to trigger any blacklist-based filter, and when you are using a self-learning filter, these strings will train it with garbage-data, which can only reduce its efficiency.