I would guess that you probably butted up against some muscle, given that the needle wasn't in to the base. You may also have nicked a venule (minute veins that run just under the skin) which would've caused a slight pooling of blood and probably the discolouration you noticed.
You may want to stay away from that site in the future, although it could've been a one-shot deal.

I've had that happen several times in the past and unless it's affecting your BG levels, I wouldn't call it a concern.
Shy