There is nothing black box about the MS stack. The .net bcl might be closed source but you can still step into and read the source. It's available under the MS-RSL license. Which allows you:

"use of the software within your company as a reference, in read only form, for the sole purposes of debugging your products, maintaining your products, or enhancing the interoperability of your products with the software"

See: http://referencesource.microsoft.com/