The Black Gobbo could Start mostly anywhere cuz hes like grombrindal but for gobbos...so i personally think he should start in place of a goblin lord on the map so i suggest two places: the black pit (so he starts near skarsnik that would be so awesome for gobbo coop :3) or let him lead the scabby eye tribe that would be good too :D sooooo to the ideas:

The Black Gobbo should be a Night Goblin Warboss with Darker clothes (i know you know that XD) but maybe instead of a spear if possible he should have a axe!

To his Faction abilitys:

40 % upkeep reduction for gobbos (obvious) like skarsnik, cuz i think he shouldnt get more or less of that

Little buffs for him and his gobbos vs dwarfs (like melee attack/ defence, weapon attack or leadership)

diplomatic relations with dwarfs -50 or more (cuz he HATES them)

it sounds maybe very strong, but he should have the ability unbreakable if he fights dwarfs! (am i the only one thinking that?) cuz like i said he HATES them

if possible....maybe a buff for him if he beats grombrindal? i dont know if its possible but that would be hella fun :D

Im not good enough in this to make a full unit card here XD but here is my take on it:

Melee Expert
Duelist
Armored
armor piercing (for the dwarfs!)

so his stats should look like this (IMO)

HP: around 2000 (depending on unit size ofc)
Armor 50 (blocks 35 percent of all missile damage)
Leadership 70
melee attack: 55
melee defence: 55
weapon damage: 450
charge bonus: 30

now the effects he should have in campaign map:

tenacity: badlands, desert and i would add mountain...cuz he hates dwarfs and it would be mean if he takes attrition dmg if he pursues them :C

hide in forest

the items:

Gromril Armor, gives him 5 percent ward save, 5 melee defence and 20 armor

the hood of night: leadership buff for goblins and trolls (why not D:?) think like 9 LS

Belt Buckle of Durzik Al Drazh: buff for goblins in melee defence thinking of 5 points (double if fighting dwarfs), diplomatic relationship with dwarfs -30 and with greenskins + 30

Gotkid's Beard: ward save 5 percent for black gobbo and leadership for goblins (again 5 points, doubled vs dwarfs)

so...let me think...what you think of my take :D