Although Unity can import and work with *.blend files directly (Blenders native file format) it’s often more useful to use *.fbx (FBX), largely because they don’t include extraneous scene data that has to be dealt with. However, because both Unity’s development environment and Blender 3D use different cardinal orientations – Z Up for Blender, Y Up for Unity – exported content is often incorrectly rotated around the wrong axis; meshes that lay flat in Blender will appear standing upright for example. To fix this meshes need to be rotated prior to export in Blender.
Design note: the orientation of a selection can be changed at export in Transform settings but not reliably. The information outlined below is a fail-safe option that sets the orientation at source forcing Blender and Unity to honour rotational data baked to the FBX file.
Rotate on X
To fix Blender exported meshes being incorrected rotated in Unity, once an Object is ready for export to *.fbx, in Object Mode select and then in Object Properties  or Transform properties of the Sidebar (N), change the Rotation value of X to -90 degrees  (negative 90°), e.g. X: -90°.
Design note: changes made to rotation are irrespective of an objects orientation in the 3D Viewport; an object lying on its back or standing upright should be similarly rotated ‘backwards’.
Important: confusingly Unity also uses the location, rotation and scale of objects at time of export so they may appear correctly when dropped into the 3D View [b] but not in the Asset Browser [a] preview and/or Transform in the Inspector [c]. It’s important to address this problem so object coordinates are consistent and universal, especially in relation to the more advanced aspects of development.
Apply Rotation & Scale
Once the necessary rotation value is set the object needs to be ‘fixed’ otherwise the orientation data exported to the FBX will retain the original information instead of the update. To do this, with the object still selected, from the Object menu  select Apply then Rotation & Scale  – Object » Apply » Rotation & Scale (alternatively Ctrl + A). This fixes and resets the rotational value back to 0°  – X now reads X: 0°. The file can then be exported to FBX and Unity.
Design note: if content needs to be edited further, once rotated and set for Unity, objects can be (re)rotated back to their original orientation (in Object Mode – they will then appear rotated on X by +90 degrees, i.e. X: 90°). Any subsequent changes to the file can then be exported without rotating again because the X value is relative to the object (local) rather than the scene (global).