The first ever add on.

[Doomonkey]

Retail Blockland? Or just Blockland.

[Ad Bot]

[Teneksi]

Retail Blockland? Or just Blockland.

Blockland itself is not a Blockland add-on.

[EndGame]

Blockland itself is not a Blockland add-on.

I think he's asking if add-ons created before the retail version was released count.

[Doomonkey]

I think he's asking if add-ons created before the retail version was released count.

Yes.

[Loopyla1]

Tier Tacticals.

[LeetZero]

Tier Tacticals.

[insert facepalm image here]

Going on-topic: I find it quite obvious one of the first Add-Ons ever, in Vanilla and Retail is a script.

[Manzaross]

Tier Tacticals.

XD
Have this 

[Killers22526]

i think the first add-on is the default add-on for the 1x1 brick

[Munkey]

I think it was a colorset.

[Cyanian]

I think it was a colorset.

As do I.

[BlackMod]

I use to think it was that stunt plane thing

Bump

[_Rave_]

Bump

Are you kidding with this stuff?

[Sabbin]

Are you kidding with this stuff?

~WhatevaGuy

[_Rave_]

~WhatevaGuy

Dun go thar 

[TheArmyGuy]

I think it is the...
Uhm.
Tank or something.
Jeep perhaps.