What is the meaning of 'uniform' in Blum's paper "On the Size of Machines"
In this context, uniformity usually means that there exists an algorithm/Turing machine computing the thing that is being called uniform. That is, in this case, that there is a Turing machine that will compute $i,j$ from (Turing machines for) $f,g$.
More formally (in the context of the paper), it means:
There is a Turing machine $B$ such that, for any total Turing machines $F,G$, where $G$ has infinite range, $B(F,G)=(i,j)$ such that...