La blava combina millor amb el seu abric blanc.

Yes. In la blava, the adjective blava is being used substantively—that is, like a noun.

Catalan often does this when the noun is understood from context. So la blava can mean something like:

• the blue one
• the blue item
• the blue garment

The feminine article la tells you the omitted noun is feminine, or at least that the speaker is referring to it as feminine.

For example:

• la blava = the blue one (feminine)
• el blau = the blue one (masculine)

So the sentence does not need to repeat the actual noun if it is already clear.

Because the adjective agrees in gender and number with the thing it refers to.

Here the implied noun behind la blava is feminine singular, so the adjective must also be feminine singular:

• masculine singular: blau
• feminine singular: blava
• masculine plural: blaus
• feminine plural: blaves

This is standard adjective agreement in Catalan.

The subject is la blava.

So structurally, the sentence is:

• La blava = subject
• combina = verb
• millor = adverb
• amb el seu abric blanc = prepositional phrase

In other words, the blue one is what goes/matches better with the white coat.

Because the subject, la blava, is singular.

The verb combinar here is conjugated in the third person singular present:

• jo combino
• tu combines
• ell/ella combina

Since la blava is a singular subject, Catalan uses combina.

If the subject were plural, you would get:

• Les blaves combinen millor... = The blue ones match better...

Here millor means better and modifies the verb combina.

So:

• combina millor = matches better / goes better

Catalan commonly uses millor as the comparative of bé in this kind of sentence, just like English uses better instead of more well.

So:

• bé = well
• millor = better

Using més bé here would not be the normal choice.

Because combinar amb is the normal way to say that one thing matches/goes with another thing.

So:

• combinar amb alguna cosa = to match/go with something

Examples:

• Aquesta faldilla combina amb la camisa.
  = This skirt goes with the shirt.

• El vermell combina bé amb el negre.
  = Red goes well with black.

So in your sentence, amb el seu abric blanc tells you what the blue item matches with.

Seu means his/her/its/their depending on context.

Catalan possessives like seu / seva / seus / seves agree with the thing possessed, not with the owner.

Here:

• abric is masculine singular
• so the possessive is seu

That does not by itself tell you whether the owner is male or female.

So el seu abric blanc could mean:

• his white coat
• her white coat
• sometimes even their white coat, depending on context

Catalan often leaves that ambiguity unresolved unless the context makes it clear.

Because in Catalan, adjectives usually come after the noun.

So the normal order is:

• abric blanc = white coat

Not:

• blanc abric (this would sound unusual or literary in most contexts)

The structure is:

• el = the
• seu = his/her
• abric = coat
• blanc = white

Catalan word order here is very natural and standard.

Yes. Blanc agrees with abric in gender and number.

Since abric is masculine singular, the adjective is also masculine singular:

• abric blanc = masculine singular
• jaqueta blanca = feminine singular
• abrics blancs = masculine plural
• jaquetes blanques = feminine plural

This agreement is a basic feature of Catalan adjectives.

Yes, very easily. Without more context, la blava could refer to any feminine noun that has already been mentioned, but in real usage it often refers to a garment or item whose noun is omitted because it is obvious.

For example, if several jackets or shirts are being discussed:

• La blava combina millor amb el seu abric blanc.

That would naturally mean:

• The blue one matches better with his/her white coat.

So Catalan often avoids repeating the noun when the adjective plus article already identifies it clearly.

In this sentence, it is used more like to match or to go well with.

That is a very common use of combinar in Catalan when talking about:

• clothes
• colors
• decoration
• style

So although combinar can sometimes resemble English combine, here the natural sense is:

• to match
• to go with

That is why combina millor amb... is best understood as matches better with... rather than combines better with... in a more literal English sense.