Why do my sprites have a dark shadow/line/frame surrounding the texture?

Question

I'm starting OpenGL with Apple's GLKit hand I'm having some trouble to get my sprites displayed properly. The Problem is that they all are surrounded with thin dark lines. The screen shot below shows two rectangles with a png image textures containing transparency (obviously).

The black shadows, surrounding them are definitely not part of the pngS. The green png is done without anti-aliasing the blue one has an anti-aliased border. The black border is also apparent if I draw only one sprite.

Te relevant part (hope so...) of code is:

//render the scene
-(void)render
{
    glClearColor(69./255., 115./255., 213./255., 1.);
    glClear(GL_COLOR_BUFFER_BIT);
    [shapes enumerateObjectsUsingBlock:^(AAAShape *shape, NSUInteger idx, BOOL *stop)   
     {
         [shape renderInScene:self];
     }];
}

//creating and storing the effect inside shape class
-(GLKBaseEffect *)effect
{
    if(!effect)
    {
        effect = [[GLKBaseEffect alloc] init];
    } 
    return effect;
}

//rendering the shape (including effect configuration)   
-(void)renderInScene:(AAAScene *)scene 
{
    //TODO: Storing vertices in Buffer
    self.effect.transform.projectionMatrix = scene.projectionMatrix; 
    self.effect.transform.modelviewMatrix = self.objectMatrix; 
    if(texture)
    {
        self.effect.texture2d0.enabled = GL_TRUE;
        self.effect.texture2d0.envMode = GLKTextureEnvModeReplace;
        self.effect.texture2d0.target = GLKTextureTarget2D;
        self.effect.texture2d0.name = texture.name;
    }
    [self.effect prepareToDraw];

    if(texture)
    {
        glEnableVertexAttribArray(GLKVertexAttribTexCoord0);
        glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, 0, self.textureCoordinates);
        glEnable(GL_BLEND);
        glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    }

    glEnableVertexAttribArray(GLKVertexAttribPosition);
    glVertexAttribPointer(GLKVertexAttribPosition, 2, GL_FLOAT, GL_FALSE, 0, self.vertices);
    glDrawArrays(GL_TRIANGLE_FAN, 0, self.vertexCount);
    glDisableVertexAttribArray(GLKVertexAttribPosition);

    if(texture)
    {
        glDisableVertexAttribArray(GLKVertexAttribTexCoord0);
        glDisable(GL_BLEND);
    }
}

Any ideas anyone? Thank you.

Answer 1

Come on stackoverflowers, 14 hours and no answers ;-). On gamedev it took David 14 minutes to give this great answer. Vote him up!

Answer 2

This worked for me:

glEnable( GLES20.GL_BLEND );
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);