Hello and welcome to the Chrons!
I just stopped in to agree with what has already been said. There's nothing wrong at all with blending the two. It's
your story, and if that story involves a knight implanted with cybernetic enhancements facing down a dragon summoned by a 600 year old warlock,
then so be it!
What I will add is that (IMO) you shouldn't really worry about what genre your story fits into. When all is said, done, and sent off, that will be for your agent (and readers) to decide.
I think of the Sci-Fi/Fantasy blend like
Star Wars and
Star Trek. I have heard the arguments all my life that
Wars is fantasy and
Trek is sci-fi.
And yet with the technological marvel of lightsabers and blasters (not to mention the Death Star) in
Star Wars and the fantastical abilities of the being "Q" from
Next Generation, I tend to think the two have more similarities than differences.
My point being that one is still (generally) classified as fantasy, and the other sci-fi. Regardless, that doesn't detract from the enjoyment of either.
If your story is structured, presented, and written well, then it will find an audience. And let's be honest, will it really matter what genre they say it fits?
Besides that, I believe slapping a genre definition on your project hinders you creatively, as the tendency will be to struggle in desperation as we try to make it fit within the confines of that genere's "box". That is (IMO) a difficult temptation to escape.
One of my current WIPs (I think) doesn't really fit into a genre. If I had to put a label on it, I would call it Psychological/Supernatural/Horror/Christian/Dark/Urban Fantasy.
If I tried to make it fit into a box, I would have walked away long ago!
Apologies for the rambling. Bottom line - no, there's nothing wrong with it. Yes, it can (and does) work.