Walking under a ladder is a bit silly. You don't know if a pot of paint, a bucket of dirty water or a roof tile will land on you but that isn't why it's unlucky. A ladder, when lent against a wall creates a triangle and according to the old Catholic church, represented the triquetra of the Father, the Son and the Holy Ghost, so walking underneath a ladder broke this triquetra and marked you as working with the devil. Never even a truly evil devil worshipping witch of the medieval period would risk this as they would be tortured and killed.
A black cat crosses your path. In the UK, a black cat crossing your path is lucky, but in America, its the opposite. Personally, if a black cat crosses my path it's lucky, and it's even luckier if they come over for an ear skritch and stroke. I think this one has something to do with witches again. I blame the Puritans and witchfinders of the medieval ages for this one. Witches, (old ladies really), often had cats to keep the rats and mice at bay, but they were often regarded as familiars, as something the witch would transform themselves into to escape from religious folk. You can find more about these superstitions in the following tow books.
The Dictionary of Omens and Superstitions compiled by Philippa Waring
Encyclopedia of Superstitions by E & MA Radford
The Dictionary of Omens and Superstitions
Encyclopedia of Superstitions www.worldofbooks.com/en-gb/books/m-a-radford/encyclopedia-of-superstitions/GOR004896714?keyword=&gclid=Cj0KCQjw0YD4BRD2ARIsAHwmKVlRbac6B_HH5mKeIdDdxz1atf8a4RLe5DycRtTWqbafL15oLxkIjbIaAjFIEALw_wcB
I have no idea why the hyperlink is not working with the second book. I have tried connecting it about three times to the text, but it's not working. Sorry.