Add-ons based on Hulse's performance and whether the Blades stay in the Premiership could take the fee up to £3m and Blackwell has already re-invested a portion of that with Luton captain Kevin Nicholls completing a £700k move.
Blackwell said in the Yorkshire Post: "In an ideal world, you always keep your best players and then add to them. But it isn't an ideal world. I wish Rob all the best, he has been a great lad and a good ambassador for the club. I hope he goes on to do well in the Premier League with Sheffield United.
"I have to generate money because the club has lost the parachute payment but this year will still have to pay the £9m wages of people who aren't even still here.
"At the end of this season, the club will be in great shape. But we have to get there and I have to keep the club going forward on no money. That is why I have to wheel and deal to work with what I have got.
"My track record here has seen people come in for a lot less than what they have left for. We have doubled our money on Rob Hulse. Matthew Spring and Clarke Carlisle came in for nothing, as did Danny Cadamarteri. Simon Walton also did not cost us a penny and yet we brought in around £1m.
"It is pleasing to do the business right. My responsibility, and I will never forget this, is to look after the finances of this club and not just the football. So, when a situation comes along where I can help the club financially, then I have to do it."