I love the pics that Elle M posted, especially the first one!
Denim cut offs with black opaques are cool if you have thin legs, in my opinion, and preferably long legs too. I don't have long legs, but I do have (reasonably) thin legs, so I can pull off that look quite well.
And, as others have said, the shorts ought to be short! As you're wearing tights, then you won't need to worry about people telling you that your shorts are too short and that you're flashing too much flesh.
When in Rome in March last year with some friends, I was wearing denim shorts that sat just above my knee (they were hideous, but I was running out of clean clothes as the holiday was coming to an end). It typical weather for Rome in March; sunny but not really hot, and ocassionally windy. However, it started raining and my bare legs became really cold. My friend had a spare pair of black opaques in her bag and I went to put them on. I wore them under the shorts (obviously) and they looked awful! I didn't realise it at the time. I was wearing black Converse lo-tops, because we visiting all of the typical tourist-y attractions and were walking a lot, and it just looked so bad! I looked back at the photographs of that day and thought, "OH MY GOD, WHAT WAS I WEARING?!"
Oh the shame. So, I have learnt that one should only wear shorts and tights when one's shorts are short! And I mean short!