Blackwell was only approached about the post on Friday but before the day was out he had agreed to become assistant to new Leeds boss Peter Reid.
"It is something I just did not envisage happening so quickly," he said.
"I woke up this morning as assistant to Neil Warnock and now I've agreed to become head coach at Leeds. It's unbelievable.
"I spoke to Peter Reid and he wants me to come as head coach. My role will be as his second in command.
"I have agreed everything with them and now it's up to the clubs to sort out compensation.
"Neil is stunned really. There's no doubt about that - to lose the backbone of your coaching staff - it's a big blow to him."