Black is rich, you want tan or grey depending on the type of oil and oil mix. Most good weedeater carbs have a hi & lo mixture screw.
If it cuts the grass leave it alone, if it four-strokes and won't rev up to speed then go ahead and lean it out.
Playing with the 3 adjustments, idle, low and high can result in a non running machine if you are not familiar with them.
My suggestion, take it to a lawn care shop and see if they will explain which screw is which then adjust accordingly.
Otherwise google is your friend.
A rich motor runs cool, it is preferable to a siezed lean (hot) one.