For Drupal 7 :
$block = module_invoke(‘block’, ‘block_view’, 30);
@arg 1 : module name
@arg 2 : hook name like block_view, block_info
@arg 3 : id or delta of the block e.g 30, map-block_1
for Drupal 6 :
$block = module_invoke(‘block’, ‘block’, ‘view’, 30);
@arg 1 : module name
@arg 2 : hook name like block
@arg 3 : $op of hook_block e.g. info, view, configure
@arg 4 : id or delta of the block e.g 30, map-block_1