I don't know why you saw availability in economy on the 3rd and I didn't. Maybe I searched right after you and you were effectively blocking the last economy seat.

What's called Economy in your chart is what I called Turista above. The only difference with Blue class is that it costs more Avios and it can be cancelled for free (instead of for € 25). It is not Basic Economy. In particular, both Blue and Economy have seat selection and checked luggage included. The difference in Avios is worth more than €25, so it's normally better to book Blue if available (especially if the outbound is Premium Economy, which means you would have to pay €25 to change anyway).