Hmmm...I can never remember which is which...maybe the other users can help out...but I always thought that blue was gas and white was
oil...
I really don't think you have anything to worry about. I'm almost positive it's just a little rich. It's really not gonna hurt anything...just maybe pop and crack every now and then. A rich cylinder won't make as much horsepower as one towards the lean side....But of course you know, it's better to be rich than lean.
Now that I re-read your post, I see the part about doing it at idle...I'm gonna retract my statement about it being the intake manifold...hehehe....I'd say your carb is just a little off-tune.
What kind of carb is it? Most higher performance carbs have 4-corner idling circuits....you could probably lean it up a little bit with the mixture screw on that corner. Or....change out the idle air bleed on that corner. Let's see, you would want more air to that corner, so you would put in an idle air bleed with a bigger orifice...
I'll be the first one to tell you that carb'd engines are really finicky. It's not like a fuel-injected engine....everytime the weather changes, or it gets colder outside, you'll probably have to tweak something.