How to detect if fuse has blown?
If you can live with a bit of leakage through a blown fuse, and there is always a reasonably low value load, put one optoisolator LED across the fuse (with a resistor in series, of course, and an inverse diode across the LED wouldn't hurt). 

Otherwise you could do something like this: 







simulate this circuit &ndash; Schematic created using CircuitLab