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.