What does does a blood sample tell you about an individual's identity?

I am working on a fiction/mystery scene where blood is found in a particular character’s residence but the character themselves is missing. I would like to understand whether the law enforcement/forensics team are realistically able to identify if the blood belongs to the missing character, without having a prior blood sample or DNA of this missing character? If not what details are they able to learn from the sample eg. gender, age, physical characteristics etc?