These are the cells that UE should not consider while doing measurements, handovers or cell-reselections. These black-listed cells can be Intra-frequency, Inter-frequency or Inter-RAT cells .
The black-listing is provided in the SystemInformationBlockType4 and SystemInformationBlockType5 that is transmitted by RRC in E-UTRAN.
By providing a black-list of the cells, the UE can avoid wasting time performing measurements on that frequency and then reading the System Information only to find out that there was no point of doing this operation. At the same time the neighbour list can be reduced and the amount of cells the UE reports in measurements can be reduced. The UE can also re-select to a new cell faster because the neighbour cell list size is reduced.
The possible reasons for black listing could be that a cell is reserved for operator then the UE's should not be able to use them. Similarly if the cell is reserved for future use or reserved for private use (Femtocells?) or in a scenario where it wont be possible for the UE to reselect it (Examples are cells that are separated by a river and micro cells that are in two different skyscrapers, etc).