Suppose $\mu:X\rightarrow Y$ is a blow up of a smooth irreducible subvariety $Z$ of a smooth projective variety $Y$. Let $L$ be an ample Cartier divisor on $Y$. Let $E$ be the exceptional divisor of $\mu$. Is it true that there exists $0<\epsilon\ll 1$ such that $\mu^\*L - \epsilon E$ is $\mathbb{Q}$-ample?


It works for surfaces and blow up of a point. I don't see how to prove this for general blow up.



pull back of an ample line bundle under a blow up
Let me start with a little nitpicking:


1. What on Earth do you mean by $\mu^\*L-\epsilon E$ when you said that $L$ was a line bundle? You cannot add a line bundle and a divisor! So, let's assume that you said that $L$ was a Cartier divisor (or at least $\mathbb Q$-Cartier).
2. It looks like you are assuming that Y is smooth, or at least that it is smooth along Z, but you didn't say so. If $Y$ is not necessarily smooth, then the exceptional set is not necessarily a divisor or irreducible, so your description of $E$ as **the** exceptional divisor is totally unsubstantiated. So let's assume that $Y$ is $\mathbb Q$-factorial and that you want an exceptional effective $\mathbb Q$-divisor $E$ such that $\mu^\*L-E$ is $\mathbb Q$-ample.


With that, you can do this:  

Let $H$ be a very ample Cartier divisor on $X$ (Homework: why is there such a divisor?). Then $\mu\_\*H$ is a Weil divisor on $Y$ and if $Y$ is $\mathbb Q$-factorial, then it is $\mathbb Q$-Cartier and hence $\mu^\*\mu\_\*H$ makes sense and is a $\mathbb Q$-Cartier divisor. 


Then it is easy to see that $H-\mu^\*\mu\_\*H$ is a $\mu$-very ample $\mathbb Q$-Cartier divisor and also that it is negative effective $\mu$-exceptional.
Finally, since $L$ is ample it follows that $a\mu^\*L+H-\mu^\*\mu\_\*H$ is ample for $a\gg 0$. (One could for example argue that $aL-\mu\_\*H$ is ample (actually nef is enough) for $a\gg0$ and hence $a\mu^\*L+H-\mu^\*\mu\_\*H$ is also ample).


