Some of the ways:

• Use the settings built into native blocks already, like changing colors or using columns.
• Add a class manually to a block, or use a block variation, that has custom CSS you put in your site’s stylesheet.
• Edit the block as HTML and add inline styles. But be careful not to do anything too weird. A <span> is OK but most other elements aren’t.
• Use Dave’s ancient Better Art Direction plugin.
• Use the Blocks CSS plugin.
• Drop in an HTML block with a <style> tag, like Geoff suggests.
• Build your own custom blocks.
• Make a whole post template that is styled uniquely.

Don’t do it — it doesn’t usually age well. Buttttt it’s fun so…