The actual seeds of our Kunzea and Leptospermum species are orange-brown when ripe - never black. Do you mean the capsules? Either way pick the maturing capsules of Kunzea and Leptospermum (i.e. as soon as they start to open (on the tree) or are greyish in Leptospermum. Clip off the branchlets and put them upside down in a brown paper bag, allow to dry, then shake the stems and collect the seeds that have collected at the bottom of the bag and sow a.s.a.p. (they don't store well).