I’m not sure that here is proper place for this post.
Is any specification of j3o format?
Its not an interchange format and is only meant to be opened by the jme3 classes. So… no.
there is a way to parse some info from the header such as signature, version number and maybe a few other things, http://hub.jmonkeyengine.org/groups/development-discussion-jme3/forum/topic/j3o-format-changes/
Yes but still, do not try to mess with the binary files outside of jme, the format is bound to change and only the surrounding classes can properly reconstruct the data. Again this is not meant as an interchange format.
Also any update can change the content completly, so if whatever you use external with j3o will get broken very often.
So better use another format, or use jme to load and write to j3o.
(I’m was asking because I wanted to write a python plugin to blender for export to j3o because currently is not any way for export a .blend files in j3o with materials)
xD How about you use the .blend file importer? 
Its in latest nightly
But I’m are talking about .blend file importer in JMP.
When I’m try to import a .blend file it seems that is for blender JMP blugin not important what a version of blender was used for file creation: always ended with similar result:
-Is created a .j3o file
-Meshes in game are black, even with point or ambient light (but in blender rendering they have a propertly a UV texture
-When I’m exporting this display a similar message:
Packed texture. Reading directly from the blend file!
Loading material.
Loaded material definition: Phong Lighting
Materials name: {0}
Texture not found!
Child (domek_testowy1) attached to this node (domek_testowy)
domek_testowy: (0.0, 0.0, -0.0)--> null
Child (domek_testowy) attached to this node (Models/domek_testowy/domek_testowy.blend)
or:
Loading obejct: Camera
Camera created (W: 100, H: 100)
Loading obejct: Lamp
Importing lamp.
Loading obejct: Sphere
Importing mesh.
Reading texture from file!
Trying with: /home/marcin/jMonkeyProjects/Equisetum/assets/Models/domek_testowy/smieszek.png
Loading material.
Materials name: {0}
Child (Sphere1) attached to this node (Sphere)
Sphere: (0.0, 0.0, -0.0)--> null
Child (Sphere) attached to this node (Models/domek_testowy/smieszek.blend)
In both cases meshes are propertly imported but are black.
(If this can help I can post a .blend files if anyone can try it with blend plugin)
Yes, post a report with description and download of the file please. (In a new and properly named thread)