Blender Mesh Import issue

Try selecting the whole mesh (A Key) and switching to Edit mode. Then click "Mesh" and go to "Normals", then click "Recalculate Normals Outside"

Usually what you describe is the result of some normals on the model facing inwards. Blender sometimes makes it hard to see this (there is a button for "Show Normals" in a submenu in the edit mode). If you Show Normals, you can examine the normals in the blank spaces and Flip (under Mesh/Normals/Flip) them outward.

You may try this:

BLENDER (2.49b):

1) if your model is "closed" (like a box or character) disabled DOUBLE SIDED, if not leave it enabled

2) Make sure there are no lighting or cameras (if there are, erease them all)

3) In edit mode select your model by pressing "A" key

4) Press "W" key and in the pop up menu select "REMOVE DOUBLES" and accept.

5) Press Ctrl + N to calculate normals

6) Save it as a normal .blend file (Unity imports it automatically) pressing Ctrl + W.

Suggestions:

 * The most stable version of blender is 2.49b
 * Save your .blend files in your Project/Assets/Anyfolder so unity imports it inmmediatly after you saved your file.
 * Don't use particles, bakes or cloth simulations from blender.
 * You can RECALCULATE NORMALS only in the vertices that are wrong (that means not always is needed to select ALL vertices)

Hope it helped

I've have just encountered the problem myself all i did was press A in object mode switch to edit mode click on mesh click on normal's and press flip normal's huh so many models deleted because of this error when it was such an easy fix

hello, i always am using blender to make a quick yet good creation for my game, and i notice that problem often. i find that if i find out which face it is, and just play with it, i make new faces, delete overlapping faces, etc. basically, i don't have a solution, but until someone does, you could do what i do: manually fix it.

I actually found that you don't flip the normals outside, you just flip the normals of the faces that are facing in. I have no idea why Blender does this, but it just does.