Information about The Black Orb is mentioned in Sam's Journal.

If you haven't unlocked the journal yet you can unlock it by repeatedly clicking on Sam, the vector fish, located in The Depths until Sam gives it to you.

Hover over the spoiler below to reveal the relevant page in Sam's Journal.

You might have already figured out that you need to fish for seaweed to obtain the black orb, but you might not know how to do that.

The text on the page in Sam's Journal about The Black Orb is a bit misleading since the required strategy doesn't depend on the area you are in at all.
To catch seaweed you need to have a drop rate of 0 for all item types.
The image hidden below shows what that looks like.



When you fullfill these requirments you can catch seaweed and eventually obtain the black orb.
The chance of obtaining The Black Orb when fishing seaweed is SIKE currently not known but it should be around 1-2 by a very rough estimate.

If You don't want to figure out how to catch seaweed yourself you can go to next Section of this guide.

Empty space before the next section so you don't spoil yourself if you don't want to.

The fastest way to get 100 Seaweed is by using the Junk Rod with the Double Plug.
Using these items in together will result in drop rate of 0 for all items allowing you to catch seaweed.
You can also use the Complex Bait instead but that is SOOO much slower.

The Junk Rod


The Double Plug


Because there are differences in fishing speed depending on the area, I recommend fishing in the Backyard Pond for the fastest casting time.



Estimating the drop chance of the Black orb at 2% this should only require around 50 catches
Depending on your casting speed this should only take 25-50 seconds.

For me it took 51 catches for a total of 100 seaweed. I only used double plug for 49 of the catches.



Since it's not yet clear what the drop chance is, please comment below how much seaweed or catches it took before you got it.

Despite what Sam's Journal says I do not yet know what purpose the Black orb has as of yet.