Ah - on re reading, think the OP may be talking about the highlighted text selected by using the mouse (the .seltext property). AKA the highlighted text in my attachment above
If that is the case, then that is not an access behaviour but a windows one and one that I don't believe can be changed But it is not a simple case of being black - it all depends on the backcolor. No idea what the relationship is other than perhaps something like 'the opposite color'. Note that if you set the back style to transparent, it will become solid/normal when the control has the focus. So you may need to set the form backcolor as well.