Okay, now I think that I have understood the problem.
LetÔÇÖs analyze your code:

if(SBES_BlendindFunction == 1)
mu_t = compute_my_mu_t(...);
mu_t = C_MU_T(c,t)
return mu_t;

-SBES-BlendindFunction is okay (checked by you)
-Compute_my_mu_t is okay (checked by you)
If SBES-BlendindFunction is less than 1, then you use C_MU_T(c,t). But what is C_MU_T(c,t)? As far as I am concerned, C_MU_t(c,t) is the value that you are setting, so this line has no sense. For me (maybe I am wrong), this is like defining a function by parts and saying y=y in certain part.
You have both the RANS part and the blending function implemented. What about the LES part?
Another question: Is the blending function a real number? If yes, be careful with that, as in the if loop the function acts as an integer.