Sounds like this might be the arrow showing the direction and length of the ripple when "Ripple Edit" is selected. Try turning off Ripple Edit and see if the arrow goes away.
Yeah -sounds like the ripple arrow. Mine is grey rather than black.
When you move, insert, shorten or lengthen an event, the arrow is Vegas telling you it has noted exactly how far you have moved/inserted, and if you want to Ripple everything, or just that track, to accomodate that move, it's a simple matter of Ctrl/Shift F, or just F for that Track alone. I have Ctrl/Shift F as a single button on Contour Shuttle pro and use it all the time.
As you do this (called Post Edit Ripple) the arrow will disappear.
If you have Auto Ripple on, which I do not recommend, the arrow will appear as you drag/add. then the ripple will happen as soon as you let go of what you were moving.