Red refers to solid red and red merle. When it passes on its black gene, black pups will result. When it passes on its red gene if paired with a red gene from the other parent, the pups will be red. As you can see, there is no black gene to dominate. You can see from this that the color of grandparents or ancestors of red dogs play no part in the colors they will produce.
Red genes are all they have to pass on.
The red dog from two black parents has the same genetic make-up for color as the red dog from two red parents. Occasionally a red will show up in a long line of only black ancestry. Occasionally a dog appearing to be solid black or solid red is a merle. One or two flecks of lighter merling on a solid dog will make him genetically a merle. Diluted patches will show up as gray, rusty or brown, and are not desirable.