I have walnut tops and can confirm the black mark issue, though if you look at it more of a feature then it is not so bad. However, a quick sand down and all good as new.
As they age the colours change, which is very nice.
I have had mine for three years now and no issues with warping not even under the dishwasher.
You will need to rewax the top every 6mths or so depending on wear as the wax will wear off.
Top Tip number one – DO NOT PLACE cold cans of carling on the work top and let the condensation run down the can on to the top – you end up with pretty rings! A quick sand and re-wax and it was gone though so no biggy.