Huddersfield sold him to Blackburn for more than £8m. You're offering £9m up front. Their sell on % will only apply to profit, so they won't be getting anything out of this, unless your add-ones kick in later.
It's actually an interesting shout. He's clearly far too good for League 1, so it could be worth CH suggesting to his contacts at Newcastle that it's time to move the lad's development up a league. Better than Wilson I'd reckon.