Can poor circulation cause fingers and toes to go dark?

Certainly can: This is the whole concept of a gangrenous digit. As circulation decreases, the skin is not having blood supplied to it and it gets darker, this is the skin and soft tissue dying. Venous congestion can cause darkening color as well, raynaud's causes intermittent color changes.

Yes: It can be due to inadequate to circulation to the hands and/or feet, and the darkening or bluish color indicate inadequate oxygen supply to fingers and toes. Cyanosis can also point to other medical conditions, such as heart and/or lung issues, or vasospastic disease such as raynaud's. If the dark you are referring to is black, that might suggest gangrene and tissue death.