Slippery Jim

My inexperience with DirectX is certainly at fault here, but after several days I'm still mystified by this undesired behavior in my relatively simple 2D application:

Each time I render, I loop through several textures and draw them in a back-to-front order by, for each of them:

1) Calling PSSetShaderResources() to update the shader's single Texture2D resource to the next ShaderResourceView
2) Calling DrawIndex() on the appropriate vertex buffer segment.

I have a simple pixel shader that is really just one basic texture sampling line:

ps.color = myOneTexture2D.Sample(samLinear, IN.texcoord0);

I've enabled alpha blending so that the alpha channel controls transparency, and everything is fine with that, except:

The Phantom Lingers.

Which is to say that each sampled texture is for some reason alpha-blended with the previously sampled texture.
So for totally opaque images, it's fine. For the first texture drawn -- such as this opaque double jellyfish I've drawn onto the background -- it's fine:
 texture1.jpg   192.22K   25 downloads

But the second texture (which I've made 50% transparent) drawn onto the scene shows the first texture (the double jellyfish) blended with it. Behind it, you can see a piece of the large background jellyfish previously drawn, which is the only thing that should be showing behind Snidely Whiplash.
 texture2.jpg   61.1K   28 downloads

A third texture showing fog (also set to 50% transparency) has that entire previous texture (Snidely + the first texture) mixed with it. It again shows the background portion behind it (though you can't see it well here), like I want it to, but also the previous texture, like I don't want it to.
 texture3.jpg   44.65K   24 downloads

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So can any kind soul take a stab at telling me what's going on, here?

If I create multiple Texture2D resources in the shader, and have a separate one for each texture, then it works fine. The problem is, I want to be able to draw dozens of textures each time I render the scene, and the shader/my hardware doesn't support having, say, 50 different textures with a long sequence of "if (index == 3) texture[3].Sample..." etc.

I can't use TextureArrays because the textures have different dimensions.

I'm sure I'm missing something fundamental in the way shaders and samplers work, but for the life of me I haven't been able to discover it over days of research and experimentation. I know it's happening before the output merger stage, but I can't figure out why the Texture2D resource is apparently not being completely overriden by PSSetShaderResource(), like one would expect, and is instead somehow alpha-blended with it. Or the previous incarnation lingers, and is blended when Sampled? I can't tell, but perhaps one of you generous individuals can!

Many thanks for any insight you can offer.
James