This was his full comment, who was he quoting? Associating black people with watermelon is a racist stereotype, that's why watermelon smile is an offensive
“They say he is shortly off to the Congo. No doubt the AK47s will fall silent, and the pangas will stop their hacking of human flesh, and the tribal warriors will all break out in watermelon smiles to see the big white chief touch down in his big white British taxpayer-funded bird.”
“What a relief it must be for Blair to get out of England. It is said that the Queen has come to love the Commonwealth, partly because it supplies her with regular cheering crowds of flag-waving piccaninnies; and one can imagine that Blair, twice victor abroad but enmired at home, is similarly seduced by foreign politeness.”
His description about black people or Muslim women isn't accurate at all and he's been criticised by people in his own party about it, so it's not just about the left being angry at what he said.