I do not think reducing the summoning cost is appropriate. That would make her 'the smaller Nymph': cheaper, but not as good. So on the premise of them all being equal, I think we should buff her another way.
We cannot alter her ability if we are to keep the idea that all Nymph abilities are the alchemy spell of their element.
I think she needs an ATK|HP buff - a big one.
Grey Nymph is 7|8. Golden Nymph is 6|8.
Her stats are comparable to Green Nymphs, except Green Nymph hits a magic number for her own ability, adrenaline.
I think Black Nymph should be similarly ideal for its own ability. Make it 6|6. True, that will make it a very good vampire, but at the cost of Lobotomizing itself.