Learn Polish: Zawsze kupuję czarny chleb.

Because chleb (bread) is an inanimate masculine noun. While feminine and animate masculine nouns change their endings in the accusative case (when they are the direct object of a verb), inanimate masculine nouns like foods or objects look exactly the same as their basic dictionary form. Because the noun doesn't change, the adjective czarny (black) doesn't need to change its ending either!

Where is the word for 'I' in this sentence?

It is built right into the verb! In Polish, the verb ending tells you who is doing the action. The -ę ending on kupuję means 'I buy'. You could say Ja zawsze kupuję, but Polish speakers usually drop the pronoun ja (I) unless they want to emphasize it.

I learned the verb 'to buy' as kupować. Why is it kupuję here?

Kupować is the basic dictionary form (the infinitive). Verbs that end in -ować belong to a special pattern where the -owa- part changes to -uj- when conjugated for a specific person. To make the 'I' form, you use that -uj- stem and add the first-person -ę ending, which gives you kupuję.

Does the adjective czarny (black) have to come before the noun?

Usually, yes. Just like in English, Polish adjectives that describe a basic, objective physical trait (like color, material, or size) are placed right before the noun they describe.