There are two dimensions to Swedish adjectives. They change for number and gender, but it also matters whether the adjective is attributive (goes before the noun) or predicative (goes after the noun and after a verb).

For attributive adjectives, it's like this (I'll make it about yellow horses and houses since svart is slightly irregular):
en gul häst
ett gult hus

den gula hästen
det gula huset

gula hästar
gula hus

de gula hästarna
de gula husen

For adjectives in the predicative position, it's like this:
en häst är gul
ett hus är gult

hästen är gul
huset är gult

hästar är gula
hus är gula

hästarna är gula
husen är gula

So the forms are the same except for the ones I put in bold, which is the same case we have here. We say hästen är svart, but then we'll talk about it as den svarta hästen.

I'm kind of confused by your question, but I'll try to answer. In Swedish, adjectives change forms for number (plural/singular), gender (en/ett), and definite/indefinite. Here are the forms (also called declensions) of svart: en svart hund (a black dog), ett svart hus (a black house), svarta hundar (black dogs), den svarta hunden (the black dog), de svarta blommorna (the black flowers). So, svarta is the plural/definite form of svart.

I wrote an explanation in answer to the top comment on this page, scroll up to read it, hope it helps!