Some time ago I started using the Crypt Blacksword which is a very very strong weapon but also slow and easily dogded in pvp. So I decided that I need a small weapon to comp up with this gigantic weapon. After a lot of testing I found out that a curved sword is a nice match to the Blacksword.
As it is the core of this build i highly recommend the "Crypt Blackword" as it has high physical and dark damage to shred through most types of armors. Infusing it to dark is also a nice option espacially if you go for spells such as dark or resonant weapon. The "Gyrm Axe" and the "King´s Ultra Greatsword" are also available weapons for this build as they share their moveset with the Blacksword.
Your secondary weapon is depending on your personal preferences and playstyle. I recommend a curved sword or a twinblade as both match up very good with the blacksword. But as long as your secondary weapon is quick and able to stun your enemy for a R1 with your Blacksword everything is permitted.
Here you will see the biggest con of the blacksword. Its weight is 24 Units. This means that you do not have much equip load left for your armor. But a good armor does not have to be a heavy armor. Personally i take the Wanderers Set as it has very low weight and looks cool.
For your rings I recommend the following setup: Third Dragon Ring ( standard); Royal Soldiers Ring +2; Life Ring +2 and Clothantry Ring +2
Those Ring are actually very standaed rings but they do their job. If you want some spells with your build, the southern ritual band +2 is capable of giving you enough attunement slots but more of that in the spells section.
With spells you actually have a lot of options as there are so many of them in Dark Souls 2. You could actually put some more points into Intelligence and Faith and cut down your dexterity. But I do not recommend that as you wont be able to properly wield a twinblade and some curved swords anymore. Only getting endurance to 30 is also a thing to do, but you run out of stamina very quickly with the blacksword. You can do that but I would rather farm and use some spices.
To the spells themselves: As you can see this build does not go into attunement, so if you want to use spells you have to wear the southern ritual band +2. This ring is capable of giving you 3 Attunement slots. You can also get the black witch hat which gives you 1 attunement slot.
With 3 or 4 attunement slots and rather low stats into faith and intelligence buffs are the best option. Espacially since the blacksword deals dark damage. With resonant weapon you can deal up to 1200 damage on one hit and that is really amazing. Resonant flesh is also awesome as it boosts up your health to about 2700.
Choosing the correct spells can be tough, so at first you should look for a catalyst to use. Thats because depending on the kind of catalyst you have it a lot easier to decide what kind of spell you want to use. You either go for a staff which implies that you want to use dark weapon, dark fog and dark orb. Or you go for an chime which means that you will use resonant weapon and resonant flesh. The third option would be the black witch´s staff which is capable of using all spells. I would recommend a chime as the buffs are stronger but dark fog can also be a real struggle for your enemy.
But this is all depending on personal preferences as spells are not a necessary part of this build.
Here I will try to explain how to use this build with its weapons correctly.
The best thing to do is to get down a R1 with your Blacksword on the enemy. This maybe sounds to be easy but as the blacksword is very slow it can easily be parried. Therefore you need to hit your enemy with a L1 or a roll attack of your secondary weapon. This stuns your opponent just long enough to be hit by your blacksword. And this hurts. It hurts a lot because the blacksword is able to stunlock your opponent until you run out of stamina with simple H1R1. With 40 endurance ( 157 stamina) you should be able to land 3 to 4 hits on your enemy until that happens.