I have a ceiling with embedded glowstone. When I went above the ceiling to make a room on the next floor, I carpeted the room (blue.) The light level in the room does not dim when the glowstone is completely obscured with carpet.

In the attached image, even though the glowstone under the carpet is totally blocked, top, sides and bottom, it still shines.

If I drop another wood block on the glowstone, it's properly obscured.

Enjoy