guys, check pls in what type of network screen stays black 3g or gsm,
this issue is after call only or in regular use from time to time?
this is know problem on desire, there can be falutly mobo, if your screen dont react and stays black - only battery unplug helps and this happends when your phone trying to switch between network GSM to 3G/HSDPA or from 3g to gsm
was having this issue long time + self rebooting and sent by desire to repair, they replacing mobo and know everything work fine for me,
proximity senzor works good in many cases, you can download apps to test you sensors if the proximity sensor react correctly and changing values then is not broken and pro problem is elsewhere,
Samsung Galaxy SIII mini GT-I8190N
stock XXAMB1 JB 4.1.2 "ORS"