What is a block index in blockchain.info?
I think that this value is an internal unique index in bc.i database.

It is not possible to use txid as an index because of 2x2 duplicate transactions in blockchain

https://blockchain.info/tx/d5d27987d2a3dfc724e359870c6644b40e497bdc0589a033220fe15429d88599
https://blockchain.info/tx/e3bf3d07d4b0375638d5f1db5255fe07ba2c4cb067cd81b84ee974b6585fb468

occurred in the early stage of bitcoin

see BIP-30 https://github.com/bitcoin/bips/blob/master/bip-0030.mediawiki

UPD: ok, you are asking about block index, but my answer is for tx index. May be using straight autoincrement int64 value for database is better than int256