With purely the information given in your original post, there's no cause for concern. From your wording it seems that it had been quite a while since you checked it, so it could be sipping a little bit through the turbo, leaking an amount that doesn't show up often underneath the car, or yes, burning some in the cylinders. You did not approach hurting the engine by running it at the add mark - there's still over 6 qts. of oil in it at that point. The black smoke is normal in that circumstance, and you did the engine a service by blowing that gook out of there. You should do it several more times!

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'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".