With a break in ratio of 24:1 that color is to be expected. That ratio is heavy on oil which is great for proper engine break in.
The carbon buildup should not simply rub off with your finger, ever.
10 to 15 seconds really is not a long enough time duration to properly color the spark plug.
Riding in down the road under a load (not down hill, that's cheating yourself) for 1/2 mile is the minimum I hesitantly recommend. 1 mile is better, 2 miles are perfect.
Also, once a spark plug is heavily blackened by a very rich carburetor tune or low gas/ oil ratio the spark plug gets a permanent stain on it and getting a true color imprint becomes nearly impossible.
I recommend burning through 2 gallons of fuel before worrying about fine tuning the engine. So long as it starts without excessive struggle and runs down the road without stalling you are good enough for the break in period.
Here is an abbreviated method I have used very successfully for reading spark plugs and tuning: http://motorbicycling.com/showthread...541#post541541
NOTE: this is done after the break in period is done with a common 32:1 fuel mix or similar.