When can you release a Blackbird back into the wild?


2 weeks ago we discovered 3 naked nestlings on our decking. One started to breathe but the nest has been dislodged and the parent birds did not come back. So we took the bird in & have been caring for it ever since. We estimate the baby was approx 24hrs old when found. 

We have been researching non stop the past few weeks as we originally thought it was a Starling but now we think it could be a Blackbird. 
The bird is in a cage with space for it to fly and perch. We are wanting to release the bird as soon as it’s possible but at what age is that do-able? It’s a feisty little thing and has kept me on my toes the past few weeks. 

I have added a photo for anyone to confirm bird type too please. 

any info greatly received