I'm qualified Reiki level 2, I don't offer it as a service as such. I had an interesting conversation with my 2nd reiki teacher ( I did level 1 and 2 on two different courses at different times). I remember one teacher telling me, there is only energy or a lack of energy - I was asking her about my concept of 'dark' energy. She suggested this is only a lack of light/energy. As mentioned already, the energy is coming through, rather than from, the practitioner. The receiver will take what is needed. Its the same as distance healing (sending healing to someone at a distance, maybe in another country), the Reiki practitioner can send energy to someone they feel is in need, but if that same receiver doesn't want to take the energy, they simply won't. Distance healing/saying a prayer, wishing someone good wishes, same thing. But I don't believe they can transfer 'negative' energy to someone. I feel, if the Reiki practitioner is stuck in their own problems, they simply won't be sending any energy, due to a lack of connection with 'source'. Even then though, if the Reiki practitioner has personal issues in their life, they can still connect with source and channel that energy. In that moment/s of channelling energy, they are not focusing on their thoughts, they are just in the state of oneness.
Reiki energy, I personally feel, is the same as the love vibration, the same as the feeling of oneness when, for example, someone looks at the stars at night, looks out at the ocean, looks at a tree, whatever the subject is that opens your heart, and feels so connected (a feeling of oneness with all that surrounds them) that they've found their thoughts have silenced. It is that energy that is channelled, which is pure natural healing. The vibrations of that energy have a subtle effect on the body on all levels, which allows the various blockages to gradually dissolve.