According to the website playrust.com, in the list of items, there are 4 tier for items.
Common, uncommon, rare, and veryrare.
Pistol bullet = uncommon
5.56 ammo = rare
AK = very rare
explosive = rare
C4 = very rare.
So based on this, the C4 explosive blueprint would be as rare as the AK blueprint.
btw immediately after i download this’ monday update, i play for 1 minute and already found the C4 blueprint. i spawned next to the 1house radtown, and the second container had it. The AK blueprint took me 20 hours of farming, and the server i found it crashed and is no longer online. At the moment the only interesting bp i have is C4 but i dont have explosive.