The second and third images are the same. But, in response to your question, no, they are definitely not crude or sluttish. Don't worry! They both looks like it comes just below mid thigh for the average height woman.
The black adds an elegant and classy touch and if you have nice legs, it makes sense to show it on a night out in the town or whatever. The trick is knowing when to wear it though and wait for the right time. You will definitely impress whoever you're with. That's for sure.
If you feel uncomfortable you can always throw over a cardigan or slip on some knee high leggings. But honestly, you should be totally fine wearing the dresses alone.
P.S. My favorite was the 2nd/3rd image because of the extra silver detail which adds a younger, more vibrant flirtier touch. The 1st image reminds me too much of the typical little black dress than many women like Jennifer Anniston wear. I love Anniston but sometimes it's nice to have your own sleeker, edgy vibe that not hundreds of women wear.
I think they're both fine. They're not my favorites, but they're both fine, depending on what you look like. It's kind of difficult for me to judge which would be best on your body type. I think I prefer the first one, though.