Springs, in general, are heat treated which makes them look dark. You can do that several ways: use a metal blackening solution (Eastwood, they may still sell it) prior to phosphating; "cook" the springs longer in the phosphoric bath; use a stronger phosphoric mix in the cooking bath. Remember this if you do your own phosphating, the bath temperature must be over 190F, or you're going to wipe off the phosphate.
(Note the use of - you, your, you're, in the same sentence.)
Jim