Il blu si combina bene con il bianco.

Because here blu and bianco are being used as nouns, not just as adjectives. The sentence is really talking about the color blue and the color white, so Italian uses the article:

• il blu = blue, the color blue
• il bianco = white, the color white

Compare that with adjective use:

• una camicia blu = a blue shirt
• un muro bianco = a white wall

In the sentence Il blu si combina bene con il bianco, both color words are treated as things in themselves.

Here it is a noun.

In Il blu si combina bene con il bianco, il blu is the subject of the sentence, so it means blue as a color concept.

But blu can also be an adjective:

• una giacca blu = a blue jacket

So the same word can function in two ways:

• noun: il blu
• adjective: blu

Blu is an invariable color word, which means it does not change form for gender or number.

So you get:

• il blu
• la camicia blu
• le camicie blu

The form blu always stays blu.

Bianco, on the other hand, is a regular adjective when used adjectivally, so it changes:

• bianco
• bianca
• bianchi
• bianche

In your sentence, though, il bianco is a noun meaning the color white.

Here si combina means goes well together with, matches, or combines well with.

So Il blu si combina bene con il bianco means that blue and white are a good combination.

The verb is combinarsi, which in this context means something like to go together well.

Not really in the literal English sense.

The si is part of the Italian verb combinarsi, which is often used as a pronominal verb. In many cases like this, it does not sound truly reflexive to Italians the way wash oneself does in English.

So it is better to understand si combina bene con as a whole pattern meaning:

• goes well with
• matches well with

Do not translate it word for word as combines itself.

For this meaning, si combina is the better choice.

Without si, combinare usually sounds more like to combine, to arrange, or to put together something, often with a more active or transitive sense.

For colors, clothing, or design, Italian more naturally says things like:

• Il blu si combina bene con il bianco
• Il blu si abbina bene al bianco
• Il blu sta bene con il bianco

So for a learner, it is safest to keep si here.

Because bianco is being used as a noun, meaning the color white. In Italian, nouns usually need an article, so con il bianco is the natural form.

Compare:

• con il bianco = with white, with the color white
• una maglia bianca = a white sweater

English often omits the article in places where Italian keeps it, so this is a very common adjustment for English speakers.

Bene is an adverb meaning well. It modifies the verb phrase si combina.

So:

• si combina bene = combines well / goes well together

Its position is normal here: after the verb.

You can also expand it:

• si combina molto bene con il bianco = goes very well with white

Yes, it is correct and natural. But there are also other very common ways to say the same thing.

For example:

• Il blu si combina bene con il bianco.
• Il blu si abbina bene al bianco.
• Il blu sta bene con il bianco.

All of these are natural, though si abbina bene and sta bene con are especially common when talking about colors, clothes, or style.

When Italian uses many color words as abstract nouns, they are often treated as masculine singular:

• il blu
• il bianco
• il rosso
• il verde

So in this sentence, il blu and il bianco are both masculine singular nouns referring to colors in general.

But when those same words are used as adjectives, agreement depends on the noun they describe:

• la parete bianca
• i pantaloni bianchi
• la giacca blu
• le giacche blu

So the masculine article here does not mean that every noun involving color is masculine; it just reflects how the color word is being used in this sentence.

A simple English-friendly guide is:

• blu ≈ bloo
• bianco ≈ BYAHN-ko

A few helpful notes:

• blu is one syllable.
• In bianco, the stress is on bian.
• The bia part is not pronounced like English bee-an; it is smoother, more like bya.

So the whole sentence sounds roughly like:

eel bloo see KOM-bee-nah BEH-neh kon eel BYAHN-ko

That is only an approximation, but it is a useful starting point.