If I understand your effect correctly, I think it can be done manually with putpixel & getpixel(make sure you've acquired the bitmaps, and following them the way they're allocated in memory), by making some of the color coefficient to be based on surrounding pixels.

Append: If there won't be much of this effect, it won't be much of a penalty on for your frame rates..

For bloom and blur you'd be better off using either HLSL (that's using DirectX, so it's probably not what you want) or GLSL, which are basically shader languages (so, hardware only). I guess you can emulate bloom in software using getpixel/putpixel, but you'd need to come up with a clever way of doing things.

Basically, bloom just brightens the pixels that are already bright (above a certain treshold) and just leaves every pixel below the threshold be. So yeah, you can do that, but it might be slow, as you're probably going fullscreen with this, and as such, you'll need to operate on the entire screen area in the buffer, and then blit the buffer to the screen. Not really hard to do, but it might (I really have no idea) be slow.

And I second FBlend. It's a great library, and it comes in handy if you're using the old Allegro branch.

A box filter is dirt cheap, and costs the same no matter what size.