A 30-year-old cold case murder investigation ended Thursday with a conviction. Seventy-one-year-old William Doyle Magness was found guilty by a Hopkins County Jury, after about 15 minutes of deliberations, of murdering Geraldine Blackmon at her home in rural Hopkins County. Investigators say she was shot in the back through the door of her house. The case was recently reopened by Hopkins County Sheriff’s Investigators with assistance by Texas Rangers. The crucial evidence, a gun barrel matched by forensics to a bullet found at the crime scene was found ini Magness’s possession at his home in Midlothian. The punishment phase of the trial begins this morning…