When you set the BackgroundColor of a static text box to "none", a black patch appears rather than transparency.
As a workaround, instead of a static text box you can open GUIDE and add a Axes in its place. After adding the axes, right click on the axes and select "property inspector"; scroll down to "Visible" and de-select the checkbox (visible = off).
Save, and close the GUI. Open the m-file and go to the opening function ("..._OpeningFcn"). Within the opening function you can set the text and specify the axes handle in the first input: