what's the point of alpha blending mode?

I am testing with different blending mode but I don't see any difference.

Is this a limit of rendering or something I don't understand?

My purpose is to have one circle showing the background image(which the gradient color).

The black big circle is filled with non-transparent color but I want to make the small circle can be transparent and showing the background image.

function onUpdate(dc as Dc) as Void {
		View.onUpdate(dc);
		var height = dc.getHeight();
		var width = dc.getWidth();
		var x = width / 2;
		var y = height / 2;
		var r = height / 2 - 20;

		var bufferedBitMap = Graphics.createBufferedBitmap({:width => 390, :height => 390,
			:alphaBlending => Graphics.ALPHA_BLENDING_FULL
		});

        // draw something on the layer
        var ldc = bufferedBitMap.get().getDc();

		ldc.setFill(Graphics.createColor(255, 0,0,0));
		ldc.fillCircle(195, 195, r + 10);

		var attr = Graphics.ARC_COUNTER_CLOCKWISE;
		ldc.setPenWidth(6);
		// dc.setColor(Graphics.createColor(64, 255,255, 255), Graphics.COLOR_TRANSPARENT);
		ldc.setFill(Graphics.createColor(64, 255,255, 255));
		ldc.setStroke(Graphics.createColor(64, 255,255, 255));

		ldc.setBlendMode(Graphics.BLEND_MODE_SOURCE_OVER);
		// dc.setColor(Graphics.COLOR_DK_GREEN, Graphics.COLOR_LT_GRAY);
		ldc.drawArc(x, y, r, attr, 0, 90);
		ldc.fillCircle(x+50, y+50, 30);

		ldc.setBlendMode(Graphics.BLEND_MODE_ADDITIVE);
		ldc.drawArc(x, y, r, attr, 90, 180);

		ldc.fillCircle(x+50, y-50, 30);

		ldc.setBlendMode(Graphics.BLEND_MODE_SOURCE);
		ldc.drawArc(x, y, r, attr, 180, 270);
		ldc.fillCircle(x-50, y-50, 30);


		ldc.setBlendMode(Graphics.BLEND_MODE_MULTIPLY);
		ldc.drawArc(x, y, r, attr, 270, 360);
		ldc.fillCircle(x-50, y+50, 30);

		ldc.setBlendMode(Graphics.BLEND_MODE_SOURCE_OVER);


		dc.drawBitmap(0, 0, bufferedBitMap.get());


    }