El plumaje parece negro.

Yes. Plumaje is a collective noun: it refers to the whole covering of feathers as one unit, so it is grammatically singular.

That is why the verb is parece and not parecen.

A useful comparison is:

• las plumas = the individual feathers
• el plumaje = the plumage / feathering as a whole

Because plumaje is being used as a normal noun, and in Spanish nouns usually need an article unless there is some other determiner or the sentence structure clearly allows omission.

So:

• El plumaje parece negro. = natural
• Plumaje parece negro. = not natural in normal Spanish

In context, el plumaje can mean:

• the plumage of a specific bird already being discussed, or
• plumage as the feature being described

Parece is the third-person singular present indicative of parecer.

The subject is el plumaje, which is singular, so the verb must also be singular:

• el plumaje parece
• los plumajes parecen

Parecer means to seem or to look/appear. It describes appearance, not necessarily objective reality.

So:

• El plumaje parece negro. = it looks black / it seems black
• El plumaje es negro. = it is black

The first leaves room for uncertainty, lighting effects, distance, or impression. The second states the colour as a fact.

Because parecer can be followed directly by an adjective.

This structure is completely normal in Spanish:

• parece negro
• parece grande
• parece viejo

So negro is describing how the plumage seems or appears. You do not need ser or estar after parece.

Because negro has to agree with plumaje, and plumaje is masculine singular.

So the adjective must also be masculine singular:

• el plumaje negro

If the noun were different, the adjective would change:

• la pluma negra
• los plumajes negros
• las plumas negras

Yes, but it changes the focus.

• El plumaje parece negro. = the bird’s feathering/plumage as a whole looks black
• Las plumas parecen negras. = the individual feathers look black

So plumaje sounds more like a description of the bird’s overall appearance, while plumas points more directly to the feathers themselves.

In most of Spain, parece is pronounced approximately pa-RE-the, because c before e is usually pronounced like the th in think.

So:

• parece ≈ pa-RE-the

In much of Latin America, and in some parts of southern Spain, it would sound more like pa-RE-se.

Also:

• the r in parece is a light tap, not an English r
• g in negro is a hard g, like in go

Yes. In Spanish, negro is the standard everyday word for the colour black.

In this sentence, it is completely neutral and ordinary because it is simply describing colour. English speakers sometimes pause because of associations in English, but in Spanish negro is the normal basic colour term.

It can be either, depending on context.

Most often, El plumaje parece negro would refer to a specific bird or animal already being discussed.

But Spanish can also use the definite article in more general statements, especially when talking about a class feature. Context tells you which meaning is intended. Without more context, a learner will usually understand it as referring to a particular animal’s plumage.