Photoshop is not automatically sampling the image background when you enlarge the canvas. The new canvas area fills with your currently selected Background color in Photoshop.

So if you want the added space to be white, black, or blue, first set the Background color swatch to that color, then increase the canvas size. If it turned black, your Background color was likely set to black.

This works with any color, but matching a photographed background is harder than it looks because many real backgrounds are not a perfectly uniform color. Filling with a single solid blue may look artificial if the original background has gradients, texture, or lighting variation.

If you want the extended area to blend more naturally with the image, enlarge the canvas, select the empty area, and use Edit > Fill > Content-Aware Fill.