Without more info we certainly can't diagnose this problem. You definitely need to call a reputable company and get them to look over the unit and see what's going on.
Some observations:
1) for an evaporator to freeze, it has to be below 32F, , if they checked the charge, how did they do it? Obviously the low side is too low. Could be low charge, a restriction and lots more. TXV failure?
2) the black goo looks like degraded compressor oil can look, but I dunno, obviously the compressor is working or it wouldn't get colt enough to freeze
Some observations:
1) for an evaporator to freeze, it has to be below 32F, , if they checked the charge, how did they do it? Obviously the low side is too low. Could be low charge, a restriction and lots more. TXV failure?
2) the black goo looks like degraded compressor oil can look, but I dunno, obviously the compressor is working or it wouldn't get colt enough to freeze