Can't help you with the granite/bluestone sorry, but I'm trying to visualise the finished result. To me having this type of combination would have your benches fighting for attention. I would be inclined to have the timber as the star attraction and keep the back bench in the background with a plain and simple material that will compliment your timber.

Although I'm not a professional in the matter, I've worked with enough bench tops to know that its good to find a happy medium when it comes to the thickness (or thinness) of bench tops. The thinner the top the more prone it is to cracking, instability etc. Not saying you have to get the thickest bench top around, but talk to a few professionals about what the daily usage looks like. How its being affixed, ie if you have a waterfall bench top etc, makes a lot of difference too.

A darker benchtop in the scullery would look fine and more importantly suit your lifestyle. Go for it.

Sorry that should have read "microcement". Thanks.