This would be a very literal umbrella. The English word coming from the Latin word umbra, meaning shade (the -ella is a diminuative suffix... so "little piece of shade"). The weather of England made sure that the secondary use as rain protection quickly overtook the meaning.
Maybe Rumia could solidify darkness if her ribbon was removed.
She probably could do it without her ribbon/amulet, but she might not be smart enough to do it to hasn't actually thought about, OR isn't powerful enough to actually solidify darkness. Then again, she may be able to do it without the amulet.