I understand that Blood Sun will work retroactively for lands on the field and will change lands as they enter. Let's say I have Teferi's Isle on the field. I play Blood Sun while Teferi's Isle is phased out, when it is phased back in, does Blood Sun retroactively affect it although it never ETB?
I would think it loses phasing but I'm starting to second guess myself and would like confirmation.
Blood Sun doesn't work retroactively, it works constantly. The verb "to lose" may mislead you in thinking that a land losing its abilities is an event, a one-shot effect that happens at a given point in time, but it's rather a continuous thing that's true of all lands as long as Blood Sun remains on the battlefield. Read Blood Sun as "Lands have no abilities except mana abilities".
Teferi's Isle will start getting affected by Blood Sun as soon as it phases in and from that point on, it doesn't have phasing.
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Blood Sun's second ability is a static ability, so it is always in effect. It doesn't care about whether, when, or how lands enter the battlefield, just that it is on the battlefield. Your Teferi's Isle will lose phasing as soon as it phases back in.
I would think it loses phasing but I'm starting to second guess myself and would like confirmation.
Teferi's Isle will start getting affected by Blood Sun as soon as it phases in and from that point on, it doesn't have phasing.