When is a blow-up Cohen-Macaulay? Let $X$ be a smooth projective variety and $Z$ a closed subscheme. Let $X'$ be the blow-up of $X$ with center $Z$. 

Under what conditions on $Z$ is $X'$
  Cohen-Macaulay?

In the case $Z$ is non-singular, the blow-up $X'$ will also be non-singular, so in particular CM. At the other extreme, any birational morphism is the blow-up of some ideal, so if $Z$ is horrible, there is no hope of having Cohen-Macaulayness. 
I'm sure this question has been studied in the literature before and I'd be interested in references for sufficient conditions when $X'$ is CM. The case I find most interesting is when $Z$ is a locally complete intersection.
 A: The local complete intersection case is actually straightforward:

Claim
  Let $X$ be CM and $Z$ an lci subscheme. Then $Bl_ZX$, the blow-up of $X$ along $Z$ is also CM.

Proof: 
Let $\pi:Bl_ZX\to X$ denote the blow-up. Clearly, $X\setminus \pi^{-1}Z\simeq X\setminus Z$ is CM.
Since $X$ is CM, so is $Z$. 
Since $\mathscr I$, the ideal sheaf of $Z$ is locally generated by a regular sequence, the sheaf of rings $\oplus \mathscr I^n/\mathscr I^{n+1}$ is locally a polynomial ring and hence $E=\pi^{-1}Z\to Z$ is a $\mathbb P^r$-bundle where $r={\rm codim}_X Z-1$.
Since $Z$ is CM, so is $E$. Finally, since $E$ is a Cartier divisor which is CM, so is $Bl_ZX$ along $E$.   $\square$
(For more on this see $\S$5 of this paper.) 
A: Since Cohen-Macauleyness is a local property, we can restrict ourselves to the affine case.
So, let $R$ be a Noetherian ring, $I \subset R$ be an ideal and let us consider the so called Rees algebra
$\mathcal{R}:=    \oplus_{n=0}^{\infty} I^n=R[It]\subset R[t]$,
together with the associated graded ring
$\mathcal{G}:=\mathcal{R}/I \mathcal{R}$.
Then $\textrm{Proj}(\mathcal{R})$ is the blow-up of $\textrm{Spec}(R)$ along $V(I)$, and the exceptional divisor is $\textrm{Proj}(\mathcal{G})$.
Then your question is closely related to the following:
When is $\mathcal{R}$ Cohen-Macauley?
This problem was studied by several authors and there are many results. See for instance the paper 
Necessary and sufficient conditions for the Cohen-macauleyness of blow-up algebras by Polini and Ulrich and the references given there. 
A: Here's a slightly different idea for references.
See On Macaulayfication of Noetherian schemes by Takesi Kawasaki.  In particular, Theorem 4.1 gives a criterion for when blow-ups of certain ideals are Cohen-Macaulay.
Macaulayfication is a way of blowing up an ideal on a scheme and obtaining a Cohen-Macaulay scheme.  Macaulayfications always exist, even in mixed characteristic, as long as a dualizing complex exists.
The point of Theorem 4.1 is that when you blow-up various things (generated mostly be regular sequences, in other words maybe even close to the the complete intersections you mentioned), you still get a Cohen-Macaulay scheme.  If your ambient scheme is already Cohen-Macaulay (for example smooth), then you have a lot more flexibility in how you can choose these parameters, whichwhich sounds potentially useful to you.  
I should point out that this may not work at all (ie, nothing interesting may result), it's just an idea.
