If the other party is a blank piece of paper, it is generally not recommended to sign on such paper. If someone else doesn't intend to add the contract words to the blank paper, then this signature has legal effect, even if it is not signed for words, there is no direct evidence to prove it.
For example, there is a lot of time difference between the writing time and the newly added words, which can be recognized. You can consult a professional legal lawyer for details.