Like whips, it'll probably darken over time. If you want to dye it, a buck fifty is pretty cheap. If you DIY, then I suggest first using acetone (tate?) first to get the slick finish off the belt. Once dry it will be more ready to accept a stain. If it's a new strap with a nice finish, it won't suck up the stain unless you strip that off first.
And if you've already hit it with Pecards, then it won't be soaking in *any* stain at all. Every little pore is filled with Pecards.
I had a very light, very old holster that I darkened nicely with a dark brown shoe refinisher thing I got at Wal-Mart. The kind you use to refurbish old shoes. Looks like crap on my shoes. Looks great on the holster.