I think there was a short pilot clip made, but it never went to series. According to this interview:
http://www.comicbookresources.com/?page=article&id=182
Liefeld claims he had a deal to air it on Fox in 1995/1996, paired with Cyberforce, but Marvel comics killed it by insisting their cartoons be exclusive on the network. I'm not sure about the veracity of that, however, because they did air The Tick.
There's some stuff about his feud with Todd McFarlane in here too.
"The reason that Todd got the license from me in the first place was cause he believed I had a pick up from Fox - as I did - to have a Youngblood cartoon. Mattel put an offer on the table for me to do Youngblood toys, based on Youngblood being a cartoon on Fox in the 95-96 season. It was going to be a block. Youngblood/ CyberForce. Half-hour each. At the time, they were doing well with superhero cartoons. Marvel, wisely, at the last minute… made a deal with them that all their superhero cartoons would be exclusive with Marvel. Spider-Man and X-Men were your #1 rated cartoons, along with Power Rangers. [Marvel didn't] want to let other people in. "If we have to compete with them on the newsstand, why should we welcome them to Fox?"
I got the call from Fox saying we just signed an exclusive deal to only do Marvel cartoons. Marc and I had to lick our wounds, because our cartoons weren't happening. In the process, the Mattel deal caused [Todd to say], I'm going to match it. I signed with him."
One thing about Youngblood is that it always seemed to me to be designed as a comic/cartoon/toy line combo. I have old Malibu comics with prototypical ads for the series that predate Image that even seem to support that.