My writings tend to be a blend of multiple genre's. I know some can work well together, like sci-fi and fantasy, but are there genre's that should not be blended? Also, is it confusing to reader's when there is not a clear genre?
Putting elements of one genre into a work that is another genre is not going to confuse your readers. The real genre is what constitutes the internal conflict faced by the protagonist.
For instance, if the chief problem faced by the protagonist is that he/she is powerless in the face of evil, you have a horror story. The dressing can consist of elements of science fiction (the Alien franchise), ordinary hum-drum life (Mommie Dearest), or have supernatural elements (typical horror).
Likewise, if the whole point is that there are men who prefer honor to life, you have a western, whether it's set in outer space (Firefly) or the Old West (just about any classic western).
Fantasy and science fiction both have their own unique internal struggles (personal good and evil for fantasy, and social good and evil for science fiction). Both tales are usually dressed up with characteristic elements, but in the case of Star Wars, the tale is about the impact of personal good and evil, so even though it is packed with the trappings of science fiction, at its heart it's a fantasy story.
Naturally, there are a lot of stories whose internal struggles are a blend of the distinct genres; 1984 is both horror and science fiction.
So identify the key internal conflict in the story—start over if it doesn't have one!—and then add the dressing you want.
You can very clearly blend elements of genres. Lots of people have done it. The real question is, will it produce a work with crossover appeal? That is, will it appeal to fans of both genres?
A good example to look at here is Joss Whedon's Firefly. It is a very clear blending of elements from westerns and space-opera style science fiction. Which audience did is appeal to? Clearly it appealed to science fiction and fantasy fans. Did it appeal to fans of westerns? Not as far as I can tell.
Now imagine a romance novel set in 1850s Wyoming with a cowboy as the hero. Clearly it will borrow elements from westerns again. Will it pull a lot of Louis L'aMour fans? Probably not.
A genre is fundamentally a promise to the reader that they are going to receive a particular kind of reading experience. Fans of westerns like horses and big hats and schoolmarms in their books to be sure, but they also want a kind of rugged earnestness of a John Wayne, not the smarmy smartassery of a Nathan Fillion.
Firefly, in other words, is science fiction set on a western backlot, but it is still science fiction because it delivers the kind of viewing experience that a science fiction fan wants, not the kind of experience a western fan wants.
So, can you blend elements from one genre into another genre? Certainly. But the result had better still be squarely aimed at the reader of one genre and had better clearly give them the core elements of the experience they expect from that genre, or no one in the entire publishing chain, from agent, to editor, to publicist, to bookseller, to reader, is going to know what to do with it.
It's very well received by readers --when done well. Many popular and influential books and movies have imported elements from one genre into another. Harry Potter is Agatha Christie as YA Fantasy. Neuromancer is film noir in cyberspace. Star Wars is a fairy tale in a distant galaxy.
But if you do it poorly, it will read like a parody or a pastiche. In general, you need to understand both genres well and be respectful of each. If you're just plundering different sources for cool ideas it will feel lazy and derivative.
There is a snobbery amongst readers (and writers). This is evidenced by the sheer volume of fantasy writers on this site. When they go to the library or the book-store they rarely look past the fantasy section.
The main genre of you work is purely marketing. But be aware the readership has certain expectations of any genre - don't put anal sex in 'Christian' literature even if your story is about a prostitute finding God.
Romance crosses all genres.
However, outside of your marketing genre the sky's the limit. e.g. I have a thriller which spends a lot of its time in the fantasy genre - the MC is a heroin addict. I also have a science fiction series which spends at least three volumes 'thinking' its science fiction. Scientists spend the bulk of the story trying fix an anomaly they believe they created but it turns out they were not to blame - all that came to pass was deemed by the gods on Mount Olympus (fantasy).
With each genre comes and expectation of 'style'. Fantasy readers expect worldbuilding etc.
Example 1: "Pride and Prejudice and Zombies." Classic, or horror?
Example 2: "The Seven-Per-Cent Solution." Mystery, or sociology?
Example 3: "The Hitchhiker's Guide to the Galaxy." Sci-fi, or humor?
You can do this when they work well together.
One of my works is a romantic comedy. But it incorporates science fiction. I have to be careful to downplay the science fiction because it is really a "mechanic" that drives the comedy; the "science" is not there for its own sake.
In these kinds of situations, one genre will be dominant, and the other(s) will be subordinate. That way, there will be a "clear genre" that is not confusing. As another poster pointed out, only one genre should determine the reading experience. I was not writing my piece for science fiction fans, although I would be pleased if they read it.