I've never been in your situation, but... I'd assume your last BFP never got any darker because things weren't right with that egg (er, zygote, whatever it officially is then). If you have a viable pregnancy, the line will get darker. A blood test won't change whether it's viable.
FRERs might not be a good measure of whether or not your levels are changing. They can be fainter or darker depending on how hydrated you are, etc. I would call your doctor and see what he/she recommends. It would be good to get in for a test before you leave on your trip.