TH,
Yes a derived sketch will display w/o a minus sign and be fully defined if it is fully constrained.
Then changing dimensions of the first sketch (or parent) will in fact update the derived one also.
As an example, use a rectangular sketch only, follow the steps to create the derived and you'll end up with blue geometry. Even if you constrain a corner, the sketch will remain blue. Try to move the sketch and you'll see the it can now be tilted or move at an angle. So, one commonly missed constraint is a that which resoved or satisfies that DOF. Simply apply a horizontal or vertical relation to a leg and the sketch should be fully defined.