I'm having trouble with drawing a .png file. The image is a blurred red circle with transparency. However, the transparency only paints in black. Here's a very simple version of my program that illustrates the problem:

PGraphics pg;
PImage blob = new PImage(100, 100, ARGB);

void setup() {
  size(400, 400);
  background(255);
  pg = createGraphics(width, height, P2D);
  blob = loadImage("blob.png");
}

void draw() {
  pg.beginDraw();
  pg.image(blob, mouseX, mouseY);
  pg.endDraw();
  image(pg, 0, 0); 
}

I don't want to erase the "blob" with background. I'm basically trying to create a paintbrush program where you can load your own paintbrushes.