Is Bulugh Al-Marham an authentic book or is it based on a specific madh-hab?

It is a very good book. Ibn Hajr[1] was a muhaddith and had good understanding of the fiqh, Alhamdulillah. There are some things that you will need a scholar to further elaborate though, which you can’t understand on your own.

Bulugh Al-Marham is not specific to any madh-hab.

References: 

[1] http://www.dar-us-salam.com/authors/hafiz-ibn-hajar.htm