Maybe I just have the worst RNG ever, but I never got a seed of light to drop from the Blind Well. I think the issue was I received my second Seed form Mara during the first full curse week, so the BW would not drop what it considered to be the third Seed. Mara's chest wound up giving me both my second and third Seeds.
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You normally get a Seed of Light from the Blind Well and the first Raid Encounter. So, in your case, you got your second Seed of Light by opening Mara's chest. If you want your third Seed of Light, you will have to do the first Raid Encounter which is Kalli.