There are many ways to say it. For one thing, it very much depends on the predicate symbols available. Let $C(t)$ stand for $t$ is a car that is parked outside, and $B(t)$ be the predicate that says $t$ is black. So we want to say that there is an $x$ which is a car parked outside, and which is black, and such that for any $y$, if $y$ is a car that is parked outside, and $y$ is black, then $y=x$. Here goes: $$\exists x((C(x)\land B(x))\land \forall y((C(y)\land B(y))\longrightarrow (y=x))).$$