Where was the black smoke coming from? Tail pipes or under the bonnet (hood)?

If the smoke came from under the bonnet, then jdd is probably on the money - the harmonic balancer (the large composite pulley on the front of the crankshaft, that drives the v-belts) has probably failed. A few other folk here have had a similar failure with similar symptoms.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)