At least, it would be undetectable to visible-light telescopes, since such a sphere would completely block the star’s light. But even a highly reflective surface has to absorb some of the energy. And if you absorb energy over time, you have to re-radiate it away in order to maintain a stable temperature.
If you’re re-radiating energy, then you’re not really using 100% of a star’s energy output. Just saying.