Replacing residential expansion joints (backer rod?)?

Question:I have the black, soft, fibrous expansion joints around 2 edges of my living room floor (we ripped up the carpet to stain the concrete underneath). I'd like to replace it with something I can stain to match. I found a little bit of info about using backer rod and caulk, but it all looks very commercial - no instructions about how to do it yourself. Anybody know how or have any other ideas?

Answers:
Get some mortar mix and fill the gaps in with it..the backer rod or caulk will not hold a stain..you can get colored caulking close to the color of the stain...but it will stand out immensly..and possibly need recaulked every couple years