Usage seems to vary, but for me:

"Lego" (no preceding article) is a mass-noun for the stuff. It's not really an amorphous blob, of course, but it's being thought of that way. Like "sand" even though sand is made up of grains. "Sand keeps turning up in my shoes after that trip to the beach." "Lego keeps washing up on the shore after that container ship sank."

I call the individual bits "Lego pieces" or "Lego bricks" or "Lego blocks" or whatever.

Some people call them "Legos", whose singular would be "a Lego" or something of the kind. In that case it would be "Legos keep washing up".

I have never seen "Lego" used as an actual plural count-noun: "There are thousands of Lego in that box". Either "thousands of Lego pieces" or "thousands of Legos".

(I confess that using "Lego" as a count-noun makes my inner pedant twitch. But language is defined by usage, and it may well be that by now it's correct.)

It's likely a trademark issue. A trademark is suppose to be an adjective (a Google search. A Lego brick.)

This is because a trademark identifies a specific source or producer of an item. If a trademark is used as a noun (Kleenexes, Trampolines) it is a step toward generic usage and loss of the trademark.

Hence, the Lego group would want you to talk about Lego bricks and Lego pieces, but everyone else is fine talking about Legos. (Except my autocorrect, which wants it to be Lego's)

Lego has a guide somewhere instructing in the avoidance of "Legos" and any phrasing that makes "Lego" overly awkward in a plural situation. On my phone so hard to find the link right now.

UK English. We say Lego or Lego bricks, rather than Legos.

Same way we'd use sand & grains of sand as plurals.

> Is it an amorphous blob like an oil spill?

That's certainly how it works in the Lego Movie!

The title sounds fine to me. I would say "a Lego block" rather than "a Lego" too, similar to how I wouldn't say, "an oil". So calling it an "amorphous blob" sounds about right.

Lego is an uncountable noun.

A not great example is "sugar keeps washing up".

uncountable. That's the word I was looking for. How are Lego uncountable? It says the number of blocks right on the box!

"Collective noun" would be a better term.

It would be a wrong term.

"A pride of lions", "a pack of wolves" are examples of collective noun.

Lego does not fit that pattern.

Sugar, Bread, or water are example of uncountable nouns.

"A gram of sugar", "a loaf of bread", "a litre of water", "a bag of Lego".

You also wouldn't say "one lego"; you might say "one Lego piece" or "one Lego 2x2 piece".

I was actually racking my brain on how Lego can be an uncountable noun since it seems so eminently countable in my mind, but then I thought of the perfect example: cutlery. Cutlery is an uncountable noun of very discrete objects. I realize now that all of my confusion is just because of my wayward American upbringing.

It's been "Legos" for the past 40 years for me. Stuff trademarks.