As long as the sand is well washed and has no impurities that may affect water quality then it will be fine. Just means that the octopus may exhibit more dark colours and may even feel more at home than a lot of the very bright tanks that are set up...
Black sand should have no effect on the Octopus. Unfortunately, the black sands on the market have no water quality benifits such as buffering capabilities. Just remember to check your pH regularly and add sodium bicarbonate (baking soda) when you do water changes to raise your pH if it's down.
kalkwasser or b-ionic work wonderfully...cheap and available at your local fish store !
I am jealous that you can get black sand...I think it would be a vast improvement over the bright white stuff we are relegated to !