Hignett, 34, will train with United during the pre-season and he is hopeful he can convince everyone that he will be a useful addition to the squad. Blackwell admitted that the former Leicester midfielder could prove a useful option.
"Craig rang me and asked if he could train with me at the club," said Blackwell on the official Leeds website.
"He had a couple of offers elsewhere but said he would like to give it a go here. I could not promise him anything but I agreed to let him train with us for six weeks pre-season training, I am fine with that because I still need more options and if he can offer us one of them then that's great.
"I want to be able to look at the bench this year and know I can change games. That was a luxury that wasn't afforded to Peter Reid or Eddie Gray last year and many times you'd look at the bench and there was no option to change it. This year I am determined to make sure I have the option to change things."