Blackthorn has been roused by the discovery that Saruman is behind the corruption of the Black Huorns.
'Hoom, I need your help one more time, little ${RACE}. If my folk knew what has happened, ta-rum, I am certain they would be as roused against Isengard as I am. But the Ents are few and spread too far.
'I need you to gather my Huorns. Search the wood for acorns dropped by the Huorns, then place them at special places throughout the forest. This will tell the Huorns that I am in need.'
Huorn acorns can be found throughout the eaves of Fangorn, bordering the Broadacres.
Blackthorn asked you to gather Huorn acorns to summon his Huorns with.
There are places to summon Huorns throughout the eaves of Fangorn, bordering the Broadacres.
Blackthorn asked you to put the acorns in special places to summon the Huorns.
Blackthorn awaits your return in the eaves of Fangorn, bordering the Broadacres.
You should return to Blackthorn and let him know that you have done as he asked.