Eight feet wide or high?

Where are you located? Heat on wood can be difficult to maintain.

If you want a deep black, go with painting a door (non-wood). That will stand up better. What direction will these doors face? Will painting door inside black work with interior design?

Here is black painted fiberglass door with wood grain design. Not sure this system is still available, but any good door company or your GC can find source for you.

What type of coverage do you have at the front of the house. Double Doors are notorious leakers (water and air) and I will usually not recommend them, and if I do, there needs to be a good roof/overhand above it. In conjunction with a quality product (no box store doors)