Can I ask why April?

Probably doesn’t want to lose flowers to a frost

Yes, outside is very cold still. Frozen until then most likely.

Even in my very mild winter climate, it is too early for this to be occurring now. Blueberries are still quite dormant. Would also question how these will be pollinated, assuming they are in your contained greenhouse or otherwise indoors.

These will not produce blueberries for maybe a year or two. It was to keep them alive because they were not planted in ground last year. Above soil level over winter in my climate could easily kill them.