While running LEC i got may unreachable blackboxes, can anyone tell me how to remove these BlackBoxes..??
What do you mean by "remove"? Remove from the reporting or remove from the design?
Unreachables are a problem only if unexpected. You can do "report gate -unreach" to see why a point is unreachable (no path to an output through any sort of logic, therefore it has no effect, therefore LEC doesn't map nor compare it).
1) unused logic in RTL
2) spare gates
3) disable test logic
4) clock-gating latches (after modeling with set flatten model gated_clock)