What is the correct name for bytes which are to be signed? I have some bytes, and I sign them with my private key using ECDSA. The resultant (71) bytes are the signature. 

But what is the name of the bytes I have signed? (If this were encryption, they would be called "plaintext".)
The correct term for bytes to be signed is “message”.
Generally, it does not really matter if a message to be signed is human readable or not. Sometimes, you may also find it mentioned as “digital message”… which practically is the same and merely extends the term to explicitly hint at the fact the message is digitally stored and/or processed.
References showing the use of the term can be found at various sources. For example: scientific papers and websites talking about cryptography and/or cryptanalysis in relation to digital signatures.
For your convenience, here are a couple of quotes showing its usage (bold-italic emphasis is mine):
randomly picked, yet well-known websites

source: https://en.wikipedia.org/wiki/Digital_signature

… Digitally signed messages may be anything representable as a bitstring: examples include electronic mail, contracts, or a message sent via some other cryptographic protocol. …


source: https://support.office.com/en-us/article/Secure-messages-with-a-digital-signature-549ca2f1-a68f-4366-85fa-b3f4b5856fc6

… By digitally signing a message, you apply your unique digital mark to the message. … This information proves to the recipient that you signed the contents of the message …



randomly picked, yet well-known crypto-related papers

source: Identity-based cryptosystems and signature schemes by A Shamir - Advances in cryptology, 1985

… However, when g is known, there is a very simple way to generate any number of signatures of any message even when the factorization of n is unknown. To sign the message m, the user chooses a random number r …


source: A digital signature based on a conventional encryption function by RC Merkle – Advances in Cryptology (CRYPTO'87), 1988

… If A wishes to sign message m …


source: Group signatures by D Chaum, E Van Heyst – Advances in Cryptology (Eurocrypt'91), 1991

… If person i wants to sign message n, he chooses randomly some set r of persons (including himself); his signature will be … If person i wants to sign message m=An), he randomly chooses …


source: The exact security of digital signatures-How to sign with RSA and Rabin by M Bellare, P Rogaway – Advances in Cryptology (Eurocrypt'96), 1996

… In order to sign message M , the signer first picks a random
seed T of length …