Hello Justin,
I'm up here in Washington State,so I can only write about what I've read and nothing more.But there are some experienced Southern growers that I read about as they experiment and push for better things.
It seems to me if your plants,and I assume they are Southern Highbush,have satisfied their recommended chill hours,then they could be fertilized.
One guy,fruitnut,has published some of his work on the Orchard and Fruit forum.He lives in Alpine,Texas,which is fairly high altitude at about 4500 feet.So I imagine it get it's fair share of cold weather.But he grows a lot of fruit in a high tunnel and it appears to work well for him.
One of his postings was about the Southern Highbush Sweetcrisp,which is his favorite.I'm not totally sure what months he starts fertilizing but it's probably earlier than me.
By trial,he came up with a mixture of one tablespoon of Ammonium Sulfate to about three gallons of water,used once per week,with the rest of the time using only water.Any more than that could burn the roots.He stops this regimen when no more growth is wanted.
His potting mixture is composted rat bedding,with droppings, that comes from a free source and a little peat moss.All his Blueberries are grown in containers.
The results are pretty incredible.I'm including a link to the growth that happened in one year.Fruitnut's post is near the end. Brady

Here is a link that might be useful: Blueberries in containers