Tensor product of abelian categories

It seems the notion of tensor product of abelian categories exists naturally. Does someone know the reference of the construction?

A generalization of Deligne's construction may be found here: http://arxiv.org/abs/0911.4979. To recoved Deligne's construction one simply takes the perspective that any abelian category is a module category over $Vec$. Here we define $V\otimes N$, for any finite dimensional vector space $V$ and object $N$ to be the unique object representing the functor $N\mapsto Hom(V, Hom(N, N))$ (really internal hom).
In general let $\mathcal{M}, \mathcal{N}$ be right, left $\mathcal{C}$-module categories for $\mathcal{C}$ any tensor category. Then the $relative$ $tensor$ $product$ $\mathcal{M}\boxtimes_{\mathcal{C}}\mathcal{N}$ is defined as the unique (up to a unique equivalence) universal object for right exact in each variable $\mathcal{C}$-balanced bifunctors from the cartesian product $\mathcal{M}\times\mathcal{N}$. As such it follows that $\mathcal{M}\boxtimes_{Vec}\mathcal{N}=\mathcal{M}\boxtimes\mathcal{N}$ where $\mathcal{M}\boxtimes\mathcal{N}$ denotes the product of abelian categories defined in Categories Tannakiennes".
Yes Julian you are exactly right. In the paper you cite the Deligne product is extended to general (bi-)module categories in a way that reduces to the product of abelian categories in the case that the monoidal category involved is $Vec$. There it is also shown that certain classical formulas (adjunction of hom and tensor, for example) hold on the level of module categories.