To list out the defects and the corresponding test cases that blocked, its very easy to generate using Excel query feature in Quality center. The query has to join test instance, tests and link table to retrieve the details. The query looks like this
SELECT T.TS_NAME, TC.TC_TEST_INSTANCE, LN_BUG_ID
FROM TEST T, TESTCYCL TC, LINK L
WHERE T.TS_TEST_ID = TC.TC_TEST_ID -- Condition joining TEST (testcases) and TESTCYCL (Test instances) table
AND L.LN_ENTITY_TYPE = 'TESTCYCL' -- This condition makes sure that defects linked to test instaces only need to retrieved
AND TC.TC_TESTCYCL_ID = L.LN_ENTITY_ID -- Condition to join LINK and TESTCYCL table
AND TC.TC_STATUS = 'BLOCKED'
For more details and different SQL queries that help you play with information you want extract refer this blog link.