Legal analysis: You can sign the contract with a blue ballpoint pen, which is generally no problem, but it is recommended to use a black or blue signature pen. There is no rigid requirement for the color of the pen when signing the contract. It's valid as long as I sign it myself. What kind of signature is used does not affect the legal effect of the contract. Generally, a black signature pen is used, and the handwriting is not easy to fade for a long time.

Legal basis: Civil Code of People's Republic of China (PRC).

Article 490 Where the parties conclude a contract in the form of a contract, the contract is formed when the parties sign, seal or fingerprint it. Before signing, sealing or fingerprinting, one party has fulfilled its main obligations, and the contract is established when the other party accepts it. A contract shall be concluded in written form as stipulated by laws, administrative regulations or agreed by the parties. If the parties do not do this in writing, but one party has fulfilled its main obligations and the other party accepts it, the contract is established.

Article 119 A legally established contract is legally binding on the parties.