<p>I'm trying to alpha blend sprites and backgrounds with <a href="http://www.devkitpro.org/" rel="noreferrer">devkitPro</a> (including libnds, libarm, etc).</p>

<p>Does anyone know how to do this?</p>

It's been a
long
time since I've done any GBA programming, but as I recall, the DS supports most (if not all) of the stuff that GBA supports.
This
link has a section on how to do alpha blending for GBA (section 13.2). I don't know if there's a DS-specific way of doing it, but this should work for you.