The level is also known as the Height of a B Tree|index or the number of blocks required to go from the root block to the leaf block.
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.
The height value can be found in the INDEX_STATS view after the index has been analyzed using the ANALYZE INDEX
VALIDATE STRUCTURE command.