Blevel is the number of branch levels and differs from HEIGHT by one (It does not count the leaf block).
Most of B*Tree indexes will have a height of 2 or 3 even for millions of record. This means that it will take in general, two or three I/Os to find your key in the index which is not too bad.
See the value
The height value can be found in the INDEX_STATS view after the index has been analyzed using the ANALYZE INDEXVALIDATE STRUCTURE command.