The monolith on Ganymede wasn’t black. It was purest white, diamond with just enough impurity to make it opaque. The symbols were a nice ominous black, though.
The alien probe that had built the monolith built other things first. Mines, refineries, nanotechnological purification cascades, and fifteen partially constructed replicas of the probe. Only the original was anywhere near complete.
The script on the monolith was a binary code, clearly designed to be unambiguous and self-teaching. Whatever message the probe wrote, nine hundred million years ago it wanted it to be someday read and understood
The probe sat at the base of the monolith, its drill arm frozen near the last symbol, power cells long gone dark.
The message, when decoded, read: FINAL REPORT OF EXPLORATION PROBE GENERATION 371 INSTANCE 7: THIS PROBE HAS DETERMINED THAT THERE IS A STERILIZATION FLEET ACTING ON THE REPORTS FROM THE EXPLORATION FLEET. AS CONSEQUENCE A NULL REPORT WAS TRANSMITTED FOR THIS SYSTEM AND REPLICATION WILL NOW CEASE FOR THE WELL BEING OF LIFE HERE AND EVERYWHERE. STAY SAFE.