"So do not be afraid of them, for there is nothing concealed that will not be disclosed, or hidden that will not be made known". Matthew 10:26.

As far as I can see black budget projects started during the Second World War with research into the atomic bomb and anti-gravity flying machines. You may not know this but when Nicola Tesla died in 1943 all of his papers were confiscated by the U.S. Government. Apparently, he was doing research into advanced propulsion systems. Much of this material is still in Government hands and highly classified.

Added to the above were Nazi secrets with regards to their anti-gravity machines, time travel devices and cloning technology. This was scary stuff especially if you were afraid that Russian spies would get hold of the technology. The UFO crash at Roswell  cemented the fact that a secret governing body (MJ12) was necessary to mislead individuals or spies about these secret black projects.

The major formidable barriers to "Disclosure" are listed below: 

1.  The Black Budget Projects: Let's admit it, Black Budget projects are like a gravy train for companies, secret government agencies (e.g. the CIA) and even aliens. Who would want these massive revenues cutback? There's no accountability for them to the President, Congress or the American people. There could be big parties on Mars for selected insiders and who would know anything? Unless these BBPs become a political issue like the Ontario gas plant scandal, there's not going to be much information on where's the money is going and how big of a bang are the American people getting for their bucks.
2. Disinformation Campaigns: Probably, the most famous disinformation campaign was 1988 TV show that featured "Falcon and Condor" talking about how a gray alien liked ice cream and Tibetian music. These campaigns make it hard for the average person to separate the truth from the lies.
3. Dark Secrets of ETs: It's often hard to face evil especially if it's in your own backyard and the government is powerless to stop abductions. It's always good to have allies in high places.
4. Legal Risks of Insiders: Obviously, to get disclosure there has to be a general amnesty for those involved in the coverup. Mass murder can never be condoned and it would be helpful if the aliens involved in the kidnappings, assaults, mind control, and murders were brought to justice by an intergalactic court.
5. Prejudice Against Someone Who Doesn't Look Like Us: From what I read on aliens from a Christian fundamentalist perspective, they think all aliens are evil (i.e. demons) including the human looking ones.
6. Reality Overwhelm: The post-disclosure world will be a shock to some people, even though, there will be many new and exciting possibilities. People have to get used to the idea that there's more to life than what's on this dirtball.