Hello, and welcome to “Fixing models from Blender to Unity 101”, I will be your teacher, whoa, that’s going to far! 😛 Just call me Crash. Lets get started, first, you need Blender (of course 😛 ), and you also need Unity.
Q. Have you ever imported a model into Unity and it didn’t look right?
A. Just give up! Err…you could do that, or, follow this tutorial! There IS hope! 😀
Now, original model, with no fixing up.
Ack! Awful right? Well, we can fix this! 😀
First, open up your model in Blender, enter into “Edit Mode”, and select the following.
Now, if you look in texture mode, it looks exactly like it should SHOULDN’T! So, make sure you select all vertices, and click remove doubles button in the toolbar. Then, it should look fixed, now, if this doesn’t work, you can go and enter “Edit Mode”, choose “Face Select Mode” in toolbar below, and select EACH face manually, and click flip direction. That’s all for Crash today! Hope this tutorial was of assistance! Have a great day everyone in the great world of 3d modeling!
-Crash
Crash's Universe! 
Actually, in Edit Mode, press ‘T’ and under the sidebar, there is a ‘Recalculate’ button. Select all then press the button. In Unity, select the model under the file browser, and near the top, it will say “Normals”. Tell it to Calculate instead of import then set the smoothing angle to about 35. If the smoothing looks bad, adjust the slider up and down a bit until the model looks good to you. 🙂
Just some tips. 😉