The "rivers of blood" element to that speech has always baffled me. Powell had a mind as sharp as a razor. An huge intellect. He could take apart, supposedly intellectual opponents, in an argument, with a few well chosen words.
When he started ranting about the blood foaming like the Tiber etc. I cant imagine he didn't realise it would cause outrage, and alienate him from mainstream politics. It remains a mystery to me.

He certainly wasn't known as a racist previously, or subsequently. Someone who knew him, said that he loved South Africa, having lived there for a time, and when he saw the footage of police beating and whipping black people it brought him to tears.
Apparently he was a near neighbour of Michael Foot and regularily went to dinner at his house, where the two big, but opposite intellects would do friendly battle at the dinner table. I would liked to have been a fly on the wall, to hear those conversations.
But as said, he was certainly something of an enigma.