The photo below is my suggestion for colour of floor. Furniture should be darker - similar to below. For the rug - choose one colour from the artwork and find a similar coloured rug. Don't put a mantle over the fireplace - it could be a health hazard - bumps and bruises. Blinds - unless you want them to make a statement I think white - to match window frames would be fine.

The fading black I have seen has been on manufactured plywood type cladding. It does not look good as looks uneven and patchy. When building we changed from the cheaper cladding to solid recycled hardwood shiplap. So glad we did, as it is now greying off beautifully.

I don’t have any practical advice but love the look. If you google images for Porters Paints Palm Beach Black you’ll see some great exterior inspiration.

I agree that it can be upsetting at this stage but your timber will all darken naturally and the difference will become less obvious over time and once you have furniture and rugs in place it's likely to be one of those issues that only you and no one else will notice. And I defy anyone to not have something like this in their house which has been newly built/renovated. I know this isn't terribly helpful but I don't think ripping anything up to start again or going to great lengths to seek recompense from supplier/installer is going to help your blood pressure either (by all means raise concerns with them and ask for suggestions from them).