This is the expected behavior under the default 'print preview' settings.
If the intention is to create a text box that has no border at all, you can right-click on the text box, choose "Line Style", and then choose "None".
If this is not the intention (meaning, you want to have a white text box border):
When you open the "Print Preview" window, click on the "Color" tab on the left side of the window. At the bottom of this page, switch "Background color" from "Custom" to "Same as figure", and your white text box border should appear white.
If the border is not visible at all when you do this, maximize the "Print Preview" window or print out the figure. Sometimes if the border is too thin, it won't even show up in the preview window, even though it is actually there (the line width by default is 0.5, which doesn't necessarily show up in the preview window).
It seems that with the white custom background color (which is the default setting), MATLAB resorts to a black text box border, since that is the only way it will be visible on paper.
