The first blogs are written by the prolific Wes Lang, a highly experienced climber who has climbed all Hyakumeisan (100 famous mountains) in Japan.
1) http://hikinginjapan.com

Hiking in Japan is a guide written by Wes packed with information on mountains and trails in Japan.  He has over 100 mountains listed on his site.

2) http://www.tozantales.wordpress.com

Another blog written by Wes but with a more personal account of his various climbs and hikes.

3) http://www.i-cjw.com

This blog has some fantastic writing and some even more amazing photography. Quite inspiring stuff.

4) http://www.onehundredmountains.blogspot.com

This is another very well written blog by a Brit about the 100 famous mountains of Japan.

5) http://www.hana2009.wordpress.com

Another blog by a Brit about trail running. A fascinating account of some incredibly fast ascents up Japanese mountains.

6) http://hanameizan.wordpress.com/

Written by the same guy except this includes trails he ran on the 100 famous mountains with his dog.

7) http://www.steepdeepjapan.com

This is a backcountry skiiing site and has some snow hiking reports.

8) http://demo.inktails.com/

This contains some hiking information for Kyushu and Shikoku

9) http://tombouquet.wordpress.com/

An interesting blog about volcanoes in Japan.

10) http://www.evergreen-hakuba.com/

This is an excellent site for hiking, snow shoeing, skiing and mountain boarding in the Hakuba area of Japan.