This was in a science fiction novel from the 1970s or 1980s, I think.
There was an alien life form that was machine life, an interstellar spacecraft that traveled to new solar systems, ate asteroids, and used the material to spawn offspring. Sort of a living von Neumann self-replicating space probe. She and her offspring were quite capable of catching and devouring human-built spacecraft.
If I recall correctly, there were aliens living in the solar system's Oort cloud of comets. They were refugees who fled an infestation of the machine life in their home solar system, and were hiding in ours. Part of the plot was the aliens trying to peacefully contact us humans to alert us to the threat. The aliens had detected the machine life queen heading this way, but humans were had not spotted it yet.
The paperback cover was mostly black, with a dark planet and a fan-like spray of sparks radiating from a point.
Does this sound familiar?