It can take a few days for the line on the BFP to change and get darker at all--and I'd guess about a week or so (maybe longer--I never tested it) for the line to match the control. Hcg only doubles about every 48 hours, and as that number grows the line will get darker. I'm sure you're fine

If it will make you feel better, see if your doctor will do a series of quantitative blood tests so you can assure yourself that your levels are rising.
Oh, and Congratulations on your BFP!!!!!