If I understand you correctly, you could create a crop fx which creates black borders which match the dimensions of your already existing black borders. Then put that crop filter on an adjustment layer over all the footage. For unzoomed clips, the filter will just overwrite the existing border, for the same effect. But the adjustment layer also will put borders on zoomed clips.
Another way to accomplish that is to creat a title whre you draw the black borders with the rectangle tool. Then you just need to leave this title at the top most video track of your timeline and you will be good to go with the images below/ behind it. Since you will not apply any effect to the title clip, its drawing (black borders) will be consistent all the time and the render will be quickier too because there will be no effect to process other than the ones you put on the images itself.
When i zoom out of one of my images i loose the black border
You mean zoom in, right? Zooming out would increase the black.