Here\'s my setup
I use ir_black in Terminal.app but in 16 color mode, and it looks great. If you really prefer 256 color, I recommend iTerm2. The settings allow you to have the terminal report itself as "xterm-256" which is what's needed to use 256 colors in Vim.
Check this website for the procedure: http://kevin.colyar.net/2011/01/pretty-vim-color-schemes-in-iterm2
There is a ir_black-based Vim theme called tir_black which is better suited for 256 colors: http://www.vim.org/scripts/script.php?script_id=2777
Looks awesome!
iTerm2 also has loads of nifty features.
I hope this helps.
on testing which colors can be displayed in your terminal of choice: i just found this perl script on vim.org which dumps a list of 256 colors your terminal could possibly display... http://www.vim.org/scripts/script.php?script_id=1349
I had the same issue with iTerm and solved it by going to Settings > Profile > Terminal > Report Terminal Type and setting it to xterm-256color
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The Mac OSX Terminal.app in Snow Leopard does not support 256 colors, which is required for the ir_black theme (this is the theme I use).
Download and try something like iTerm.app (http://iterm.sourceforge.net/), and you shouldn't have a problem with colors.
Or you could use MacVim (http://code.google.com/p/macvim/)
Edit: As of OSX 10.7 Lion, the built in Terminal.app now supports 256 colors. See the comment below by Chris Page for how to achieve this.
I have been using iTerm and was shocked to find out Terminal.app doesn't support 256 colors! I recommend Bryan's answer.
However, if you ever get in a bind like this, you can change $TERM to vt100 and vim won't try to use colors. In bash (the default MacOSX shell) you set this with:
export TERM='vt100'
I've been using a nearly identical setup, except for vim, which I grab from Macports. A few years ago I found ir_black and loved it. I now use it for all vim sessions, Terminal.app, and TextMate. Getting it to work with Leopard, and then Snow Leopard was a tad hokey. But things have improved. Follow the instructions here, Making Terminal.app look great in Snow Leopard.