Is it possible to use bitcoind as a private blockchain? For development or test purpose, is it possible to setup a bitcoind as a private bitcoin blockchain?
When run with no arguments, all Bitcoin Core programs default to Bitcoin’s main network (mainnet). However, for development, it’s safer and cheaper to use Bitcoin’s test network (testnet) where the satoshis spent have no real-world value. Testnet also relaxes some restrictions (such as standard transaction checks) so you can test functions which might currently be disabled by default on mainnet.

To use testnet, use the argument -testnet with bitcoin-cli, bitcoind or bitcoin-qt or add testnet=1 to your bitcoin.conf file as described earlier. To get free satoshis for testing, use Piotr Piasecki’s testnet faucet. Testnet is a public resource provided for free by members of the community, so please don’t abuse it.

https://bitcoin.org/en/developer-examples#testing-applications

If you need absolutely private network - just made it. Run a few clean wallets on few computers, generate bitcoins and so on.. It will be incompatible with bitcoin network, but will be completely functional in your private local network.