How can I get the key of blockindex
Where did I do wrong? How can I add 'b' to the hash?


The block hashes need to be byteswapped. Instead of 000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f, you should have 6fe28c0ab6f1b372c1a6a246ae63f74f931e8365e15a089c68d6190000000000.


  But I still got "not found". Why...


This key-value pair is not always present.


  I could get some data. I think the following code should retrieve the "blk00000.dat" block file information, right?


Yes.