clearly the normal was is by deed ..... which both persons sign - with witnesses.
So as in intellectual exercise - A wakes up and finds Blackacre is now in his name, how was it done ?
Inheritance - A could be the beneficiary of B's will and when probate is granted, A becomes the owner of Blackacre - this could happen when one of the beneficiaries is so gormless they didnt check on what B owned when they were alive ( families are like that I am speaking from experience ) so there inheritance comes as a surprise.
Trust: A is the beneficiary of a trust and the trustees secure Blackacre and then the trust is dissolved. ter -daaah A will become the owner of Balckacre.
or
one of the parents has bought Blackacre in A's name ? Trust again really
One of the lawyers may be able to fill in other ways.
Let me guess: there is now a charge on Blackacre, and A who lived a blameless life not really concentrating on things is now saying 'Me No No - definitely no ! No taxes ! certainly not ! "