SELECT * FROM ALL_source WHERE UPPER(text) LIKE '%BLAH%'
EDIT Adding additional info:
SELECT * FROM DBA_source WHERE UPPER(text) LIKE '%BLAH%'
The difference is dba_source will have the text of all stored objects. All_source will have the text of all stored objects accessible by the user performing the query. Oracle Database Reference 11g Release 2 (11.2)
Another difference is that you may not have access to dba_source.