There is no such thing as dark reiki, light reiki, etc. Reiki is Reiki, period.
Reiki is divided in 3 levels. The first level is for yourself, for self-healing and deepen understanding of one’s self. Level 2 allows you to deeper the knowledge, obtaining access to sacred symbols and heal others. Level 3 is similar to level 2, but more advanced and with an extra symbol. Then you have the Master level which is if you want to initiate others in to the path.
With that said, sacred symbols are exactly that, sacred. You’re not going about defiling sacred symbols like demonic sigils, so why try it with other things such has Reiki?
Also, there’s no such thing as “inverted reiki” or “dark reiki” or whatever. Reiki is reiki. It’s energy to heal and attunment to the universe. Hope this clarifies, from a lv3 Reiki practitioner.
Also, Reiki is not a part of religion. Reiki is a technique, and actually considered an alternative therapy technique, so it has undergone clinical trials. Please, be wary and always study these things. Some people are way too edgy and come up with riddiculous and insulting concepts.