UIImage Not Transparent

I am trying to make a view that lets a user draw with their finger. I have a png made in Pixelmator of the brush. My drawRect: method looks like this:

- (void)drawRect:(CGRect)rect
{

    NSAssert(brush != nil, @"");

    for (UITouch *touch in touchesNeedingDrawing)
    {
        //Draw the touch
        [brush drawInRect:CGRectWithPoints([touch locationInView:self], [touch previousLocationInView:self]) blendMode:0 alpha:1];
    }

    [touchesNeedingDrawing removeAllObjects];
}

The brush image is a png with transparency, but when I run the app, there is no transparency. Does anyone know why, and how to fix it?

Edit: I discovered that the image is transparent, but when I call drawInRect, the image draws the transparent pixels as the background color of the view. Is there a CGBlendMode I can use to fix this?