I have not followed much on BLU, but from the little I have read it seems much like an acceleration/optimization add-on. Just as Connection concentrator (another optimization addon) is not available in Express-C version, I can understand why BLU may not be available in the Express-C version. I think (my ignorance here) you will need some seriously large datasets (beyond what Express can handle) to appreciate BLU acceleration.
Plus the latest Google Cloud and their virtualization and DB products have made the data landscape yet more competitive.
From what I have read, it is NOT an add-on, rather a part of DB2, and it's not just Express-C that BLU will not be available in, also Express, Workgroup Server and Enterprise Server. As I mentioned, BLU is said to only be available in the 'Advanced' versions.
Not only that, but it will also not be available in ANY of the Windows versions of DB2. Really disappointed in these limitations.