There is slate and then there is slate. If you use a product best meant for indoors on your patio it will blow (flake) after the first winter but a thicker local slate will stand up better. That being said one of the attractions of slate, after its colouring, is the less than perfect finish. Bear in mind also, that slate will be more expensive to lay than bluestone.

Inkognito, thank you for your response. I've spied a pile of slate (no one seems to use it here much anymore) and if I can find out a little more about the quality of it -i.e. indoor versus outdoor, etc., I'll use it. In my opinion, I just think there's no comparison between the PA Bluestone and slate. Slate just has that beautiful...... slatiness? I don't know, but I love it!