Would getting a blackout blind reduce the temperature of my server room?
I have already silvered the windows and our aircon is struggling but we are not in a position to be able to replace the aircon yet.
Sorry if this question is out of scope
It might, but probably not significantly.
Replacing the aircon really is the only option. Let me guess, it's a small room, and the aircon just can't keep the temperature low? There's probably not enough air in the room to outweigh the thermal output of the servers. So you'll get the situation where it's not possible to cool the air fast enough to keep it cool.
One possible solution is to implement some kind of hot/cold aisle isolation so that hot air stays at the back, and cold air is drawn in at the front, although this is tricky, and might not give a significant benefit.