XNA 4.0 - Why does using a RenderTarget2d cause transparency on models?

I have been working through Riemers ShadowMap tutorial and everything was going well until the section on drawing to a separate rendertarget. I am using a different model and quads but I assumed the process would still work.

If I draw the scene directly to the back buffer I get:

When I draw it first to a RenderTarget2D and THEN to the screen, I get this weird transparent effect only on the model, the textured primitives below are still rendered fine:

The RenderTarget is defined:

PresentationParameters pp = device.PresentationParameters;
renderTarget = new RenderTarget2D(device, pp.BackBufferWidth, 
           pp.BackBufferHeight, true, device.DisplayMode.Format, DepthFormat.Depth24);

And it is used in this way:

protected override void Draw(GameTime gameTime)
{
    device.SetRenderTarget(renderTarget);

    device.Clear(ClearOptions.Target | ClearOptions.DepthBuffer, Color.Black, 1.0f, 0);


    DrawScene("Simplest");

    device.SetRenderTarget(null);
    shadowMap = (Texture2D)renderTarget;

    device.Clear(ClearOptions.Target | ClearOptions.DepthBuffer, Color.DarkSlateBlue, 1.0f, 0);
    using (SpriteBatch sb = new SpriteBatch(device))
    {
        sb.Begin();

        sb.Draw(shadowMap, new Vector2(0, 0), null, Color.White, 0, new Vector2(0, 0), 1.0f, SpriteEffects.None, 1);
        sb.End();

    }
    base.Draw(gameTime);
}

What gives?

Edit 1: I resolved the transparency effect by resetting:

device.DepthStencilState = DepthStencilState.Default;

device.BlendState = BlendState.Opaque;

But now the model has a purple tint on the side away from the light. Something to do with my Shader code I presume.

Edit 2: Turns out the purple (blue) colour was a result of the window being cleared to .DarkSlateBlue and the RenderTarget having alpha transparency. Fixed with:

sb.Begin(SpriteSortMode.Immediate, BlendState.Opaque);

Answer

Worked it out:

device.DepthStencilState = DepthStencilState.Default;
device.BlendState = BlendState.Opaque;

But now the model has a purple tint on the side away from the light. Something to do with my Shader code I presume.

Turns out the purple (blue) colour was a result of the window being cleared to .DarkSlateBlue and the RenderTarget having alpha transparency. Fixed with:

sb.Begin(SpriteSortMode.Immediate, BlendState.Opaque);