It sounds like you have an interesting project in the works. I hope I can help.
I assume that your black granite countertops are polished. If so, a very contemporary look would be to go with a polished granite floor.
Black marble would be a VERY bad choice as it is very soft and the color fades very easily with wear. There are only a few black marble choices and both are not suitable for floors with high use.
The option of black slate is viable, but slate is also soft and can scratch fairly easily but with regular maintenance and proper sealers it is certainly an option. Scratches can be somewhat hidden with a matte finish sealer such as HMK-S35 Matte Finish Sealer.
If you want a very contemporary and fairly easily maintained floor then black granite is a good choice. If you choose a polished look just know that everything will show easily and you will need to dust mop daily (I’m sure you are aware if your counters are polished). Another great option is honed black granite, but this is a bit harder to source.
If it were my choice, I would choose polished black granite as I happen to like contemporary design. It is also a very “high end” look.
Best regards, and feel free to follow up with additional questions.