We just released our Transaction API. A transaction can be created in 3 steps:
1. With only an input address, output address, and the value transferred, we build the full unsigned transaction and the data to sign it.
2. You sign the transaction data (we do not store any private keys) and send us the signature.
3. We complete the transaction, broadcast it over the peer-to-peer network, and send it back to you.
So, sharing the hot wallet private key with blockcypher is a must to use blockcypher api to create, sign and send a Tx?