Mom had a seizure. xrays appear normal. doctor said its most likely from her not taking her blood pressure medication.does this seem a logical reason?

Not a real seizure: no that does not explain a real seizure. if she had a simple convulsion or abnormal tick of course it could be explain with anything, a real seizure you need a place in the brain to initiated, like a bleed a tumor, area of infarction or hypoxemia

Logical yes.: We cannot give the opinion without knowing the full history. Any time one has High BP ; he or she must be compliant to the medicine. Uncontrolled BP can lead to Stroke and heart attack. "Seizure" could be …. some type of TIA

Possibly : Severe hypertension doesn't usually cause seizures unless the patient has malignant hypertension and hypertensive encephalopathy(HE). Patients with HE have headache, nausea, disturbances of consciousnesses, seizures and focal neurological symptoms. Having said that, there are always exceptions to the usual presentation. If no other cause was found, and her BP was very high, it's possible.