To reproduce: run the attached sample project and refer to the screenshot. Workaround: ((TextPrimitive)this.FormElement.TitleBar.CaptionElement).UseCompatibleTextRendering=true; or set the RadForm.AllowTheming property to false.
The same bug happens with many other controls, see: https://feedback.telerik.com/Project/154/Feedback/Details/215772-fix-radlabel-blurred-text-in-disabled-state-when-the-usecompatibletextrenderin