Check out translucent drawing in the manual.

some code like this should do the trick:

drawing_mode(DRAW_MODE_TRANS,NULL,0,0);
// the 128 on the following line is the translucency: 0 totally translucent, 255 fully black
set_trans_blender(0,0,0,128);
putpixel(target,x,y,makecol(0,0,0));
solid_mode();

... and of course, you shouldn't do this pixel by pixel - draw all translucent stuff before going back to "normal" mode with "solid_mode()" - or use rectfill, draw_trans_sprite or similar.

regarding your other question - its using an int because that is enough - you'll have enough room to store the data that is necessary - 8 bits for all the r,g and b, and also 8 bits for alpha channel. The authors of allegro could have made a struct or something similar of it though, but I think ints are used for simplicity (speed also might be a part of it.)

To get (for example) the red channel out of a pixel (an int) use the get_r / get_g / get_b functions. More in here: (Truecolor pixel formats)

I am not an expert on the alpha-drawing stuff - but my guess is that you make a 32-bit bitmap using create_bitmap_ex(32,width,height) - and then you can draw to it with putpixel (or other primitive drawing stuff) in combination with makeacol (to make regions of the bitmap somewhat translucent) , and then using draw_trans_sprite to draw the sprite(bitmap) to the screen/other bitmap.

(sorry for not linking the commands to the manual - I am sure you can find what you are after anyhow - in the online manual or the one that comes with the allegro package).

There should be people here that knows more about it than me though.

[edit]
Found some minor problems with the commenting on this page though:

http://www.allegro.cc/manual/api/bitmap-objects/create_bitmap_ex

Looks like the same problem is in the manual in the allegro package too.
(reported to AD.)

[edit2] oops, no, I didn't mail - having problems mailing to alleg-devel... hope someone in charge sees this then.[/edit2]

[/edit]