Well, you can classify smooth cubic surfaces up to isomorphism instead. This gives rise a 4-dimensional moduli space.

This can be seen by noting that $\mathrm{PGL}_3$ acts transitively on collections of $4$ points in $\mathbb{P}^2$ in general position, so one may assume that the first $4$ blown-up points are $[0:0:1], [0:1:0], [1:0:0]$ and $[1:1:1]$. There are $2$ parameters for each of the remaining two points, hence one has a $4$-dimensional moduli space.

Doing this a bit more rigorously shows that the moduli stack of smooth cubic surfaces is a smooth $4$-dimensional Deligne-Mumford stack. It will not be a scheme as there are cubic surfaces with non-trivial automorphisms. People therefore often consider *marked* cubic surfaces: these are cubic surfaces together with a choice of numbering of the lines. The automorphism group of a marked cubic surface is trivial, and there is a fine moduli space of marked cubic surfaces which is a $4$-dimensional scheme. See for example:

A.-S. Elsenhans and J. Jahnel: Moduli spaces and the inverse Galois problem for cubic surfaces.