The reason the scab is darker, is that tattoo scabs are formed from excess ink being expelled from the skin, blood and skin cells. If your tattoo is black, this will be the reason your scab is so dark.
It is not normal for a tattoo to need touch ups so soon. If you require a touch up due to the skin not holding the color like you liked, it is safe to say that next time you want to add another piece of artwork to your collection, you find yourself another artist.
First and foremost, tattoos should NOT be scabbing in the first place. This usually means the artist has a heavy hand, is going deeper in the epidermis, and has overworked the skin.
Properly executed tattoos should for the first couple of days ooze the excess ink and plasma, and then begin to peel. It should feel exactly like a sunburn, and look like one too. The peeling should look like that one also.
Retouching so soon, with the skin tissue still not being properly healed, will cause scarring.
Tattoo touch ups are ONLY recommended on tattoos that have faded over a five year period due to normal skin and day to day activity.