You don't need your graphics mode to have an alpha channel in order to do alpha blending. Whether the 16-bit mode is 5:5:5:1 or 5:6:5 it doesn't matter, alpha blending uses the alpha from the source image not the destination display. Same goes for 24-bit screens. There is nothing in BlitzMax at the moment that uses the destination alpha channel.
Also some people's machines wil have support for 16-bit and 32-bit modes. If I set my desktop to 16-bit and run a Max2D program on it, the internals will likely be 32-bit and it will likely tell me that the color buffer is 32-bits depth, because it either dithers or downscales the colors when you do a Flip(), to convert to the desktop format. Similar to how it works for 256-color modes. Interally textures might still be stored as RGBA8888 32-bit data even if the screen is 16-bit, unless you specifically specify that you want textures stored as 16-bit data and only if the hardware supports that.
Also some people's machines wil have support for 16-bit and 32-bit modes. If I set my desktop to 16-bit and run a Max2D program on it, the internals will likely be 32-bit and it will likely tell me that the color buffer is 32-bits depth, because it either dithers or downscales the colors when you do a Flip(), to convert to the desktop format. Similar to how it works for 256-color modes. Interally textures might still be stored as RGBA8888 32-bit data even if the screen is 16-bit, unless you specifically specify that you want textures stored as 16-bit data and only if the hardware supports that.