As far as blocks are linear, you can characterize each block by using any common metrics for linear blackbox.

For example, use one of the followings for Gain.
    (1) Zin, Zout and Conventional Votage Gain with ideal voltage drive and opened load.
    (2) Full F-parameters(A,B,C,D)
    (3) Full H-parameters
    (4) Full Y-parameters
    (5) Full Z-parameters
    (6) Full S-parameters

Use either of the followings for NF.
    (1) NFmin, Gamma_Opt, rn or Rn
    (2) Input Referred Equivalent Noise Voltage and Current. Correation Coefficient between them.