On bolding: Assume that on a casual browse, the reader's eye will always leap first to headers, and to words in bold. They'll help him understand the structure of your piece, and the most important bits. Here's a simple example - the bolded opening lines give structure, and a few key concepts are bolded to stand out immediately.
Drinking MegaPop is bad for your health. Its ingredients include cyanide, pesticide, cidercide and patricide.
Drinking MegaPop is bad for the environment. Every year, thousands of adorable bunnies are killed by MegaPop executives, and used as fuel for their ozone-depletion devices. Scientists with exceptionally big foreheads contend that merely by placing a blanket ban on all production of MegaPop, they expect that garbage production in the United States would drop by 170% and every citizen would get a free Smokey the Bear lapel pin.
Drinking MegaPop is bad for international politics. The League of Foreigners Who Don't Like Us have cited the bitter breath caused by MegaPop as their #1 reason for disliking us. MegaPop has been shown to cause leaders to make irrational, unwise decisions (such as consuming more MegaPop). Remember: a nation that drinks MegaPop is a nation that cannot be trusted. Don't let us be that nation.
On blockquotes: Blockquotes are generally used for presenting quotes from other work; a quote that's more than a line or two should be blockquoted because that clearly differentiates it from the main flow of the text, from your blog post addressing the reader.
Blockquotes are similarly appropriate for other snippets that disrupt the main flow of your writing. The example I gave above for bolding is blockquoted; that makes it very clear that that section is different from the rest of the text. It makes it very clear where the example begins and where it ends. So: examples, long asides, imaginary scenes ("Oh, yeah! I could totally imagine Agent Scully meeting Captain Picard! It'd probably go something like this:...") and other such diversions can be blockquoted, particularly if they make the structure more clear.
On code snippets: only for code, or examples of computer input/output, copy-pasting from a shell command line, etc.. It's basically a blockquote formatted specially for code (which would, as per above explanation, be a disruption of the main, prose-written flow of the post).