NickHK has given you links to show how to accomplish this - I just thought
I'd add this to either add emphasis to or explain further his comment about
"adverse effects" some people may have:
http://www.access-board.gov/sec508/guide/1194.21.htm#(k)
Once you read that and figure it out, it says that to minimize risk of
inducing a siezure in some folks, your blink rate should be slower than 2
blinks per second (very slow), or faster than 55 blinks/second: so fast as to
be steady to the human eye. Some folks find it annoying, especially if
there's no way to turn it off! But I realize that your request to do it via
conditional formatting would imply that there is some way to turn it off (by
entering some specific value).