In its description of Blois, the Cadogan guide to the Loire Valley says: "St Nicholas church has retained its medieval grandeur, even if it looks a bit grim in parts." I'm not sure I agree with the grim part.
When I was in Blois Saturday afternoon, it was very hot outside and the sun was shining brightly. That kind of weather takes away some of the grim feeling old stones and dark interiors can inspire.
It also makes for very strong contrasts in photos like these. I'll just let the images speak for themselves.
It certainly wasn't dark in the church, but it was refreshingly cool compared to 35ºC temperatures outside.
L'église Saint-Nicolas also features dozens of modern, highly colorful stained-glass windows that — quoting and translating a city of Blois historical web site — "create a unique atmosphere that is bathed in blue or yellow light perfect for meditation."