In principle you could have a two black holes orbiting each other in a region otherwise devoid of matter, but in practice there is usually a large amount of mat

In principle you could have a two black holes orbiting each other in a region otherwise devoid of matter, but in practice there is usually a large amount of matter being heated to extreme temperatures in their vicinity. So, optically, it would look very dramatic even if the bare BHs don't emit light. https://en.wikipedia.org/wiki/Accretion_disk https://en.wikipedia.org/wiki/Accretion_disk A 25 solar-mass BH is about 150km in diameter, and the gravitational waves emitted won't be appreciable until the two BHs had spiraled inward to a separation of a few hundred km. You would need to be roughly 1,000 km away to experience a 1% strain during the merger https://physics.stackexchange.com/questions/235285/how-strong-were-the-gravitational-waves-that-ligo-detected-at-the-source https://physics.stackexchange.com/questions/235285/how-stron... I don't think spin is important at the order-of-magnitude level (for naturally occuring BHs).