It's possible for the footer to look very similar to the body in a sense that no clear distinction can be made between the two. But the footer isn't a place for regular content (except links) and shouldn't be used for that. It's also not right to include a collection of 50 links in multiple columns. This could expand the footer to an extent where it might become potentially bigger than the last preceding section of content and receive greater visual weight than it. It's best when the footer is minimal in content and height and has only the few most important links with additional information given on a separate page if necessary. Since footers should promote trustworthiness, dark colors (like black) might appear as good background colors at first until we realize that people probably came to look for “the small text” that is so common in different contracts. In this case black rather prevents things from being seen (unless the text is big and white). I have seen many footers with insufficient contrast between foreground and background colors, which made text exactly as hard to read as the one in contracts. If a user selects the text, he might be able to see something, but even that is not guaranteed as I saw in one case.
Another mistake is to find free space in the footer that still doesn't have links and fill it with other promotional material. I don't think that the footer is the right place for advertisements. This sounds like a regular content, so it's better if it has its own separate section, which will provide more space for a much better description of the offer. If the content hasn't convinced potential buyers, the chance that a shorter description in the footer will do it is even smaller. Such presentation disturbs the homogeneity of the footer and makes it look as if it was patched.
Since the footer is the last visible thing on the site, we can expect that people prepare to leave. We can think of it in this way: "What is the last thing we would like to say to a guest visitor before he/she leaves?" Not all designs can give a satisfactory answer here. If well-designed, this last thing can leave a lasting impression. This is the reason why some designers try to provoke a smile at the end—through a small cheerful illustration or something else—to uplift the mood of the user in a memorable way.