Grease gets black from oxidation at high temperatures. Maybe some contaminants are mixed in. What you saw is normal for a wheel bearing that needs the grease replaced.

Why are you replacing the bearings? Unless there is something wrong with the races and rollers you should just clean, regrease, and replace the seals. Bearing are usually replaced only when they go bad.

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95 E320 Cabriolet, 169K