@nipa has pointed to the correct place (Base same as Writer) which also contains the line color for the shape. Also changeable on the Drawing Object Properties
toolbar when the shape is selected. However, if possible do not use shapes on forms. With a finished form, the shape is selectable. In newer LO versions you cannot do anything with the selected item but in some older versions the shape can be moved by the user thus defeating the purpose.
If you are looking to just place a black square or rectangle around an item(s) you can use a Group Box
control without a label.