I have seen a recapitulation of those dramatic days and it seems Queen Elizabeth actually had no black items in her garderobe on the way back from Kenya to the United Kingdom.
Apparently the Queen wore a darkblue coat with a white accent, I believe a loan from one of the ladies of the Embassy (or a spouse from one of the Embassy staff members, somewhere in that direction).
When the Queen arrived in London, first someone entered the airplane with a black coat and then the Queen left the plane dressed perfectly in black. There was a picture of the Queen boarding the BOAC plane and leaving it and the difference was obvious.