I know of a case where a LIR assigned a block to an organization and revoked it a year later after the organization did not meet the standard requirements. The organization is signed on an agreement to follow the standards. The LIR revoked the IP block, but the upstream ISP continues to announce it since it is signed on an agreement with the organization to provide routing and doesn't want to risk a lawsuit from the organization. So this block is now dead in the water since it can't be reassigned to any other client since it is in pseudo-use.
In this scenario you outline, combined with your proposal of a registry announcing 'black-holing routes' -- what compels the ISP to accept and act upon the routing announcement? And how does this different situation protect them from the lawsuits you suggest below?
No ISP will risk a lawsuit by black-holing something. This has to be done by the allocation agency (ICANN or ARIN/RIPE/APNIC).
Certainly there are ISPs that black hole routes for many reasons. For example, MFNX/Abovenet black hole routes which are considered sources of spam. Others are listed at http://mail-abuse.org/rbl/participants.html. -alan