There is only one reason for proposing to build an arena/conference centre on the Black Rock site, this is because the site belongs to the Council, providing a financial benefit for the council and excusing it from making a contribution to the cost of the building.
The Council made the same mistake with the Keep. The Council donated thei site for the Keep which excused them from providing funding for the building. Once again it is not on a convenient site, but for different reasons.
Can the Council also be sure that delegates to conferences and other events will sign up for a place outside the boundaries of central Brighton with its restaurants, pubs, museums, entertainment and the Royal Pavilion? In the driving rain and stormy winds with no entertainments Black Rock does not seem such an attractive site.
The previous scheme for Black Rock fell through because of traffic problems (moving large numbers of of delgates around at the same time) could not be solved and this is still the case.