You can define any shape to be included in the list of available arrow styles. Only the form of the shape is added as a black filled arrowhead.
Use the draw functions to create the shape to be used as an arrow style.
Select the shape and choose Format - Text Box and Shape - Line.
In the dialogue box, click the Arrow Styles tab.
Click Add and assign a name to the new arrow style.
Click OK to close the dialogue box.