You have very good hearing. Most people I tell about this say they never noticed that the M reeds for the black keys sound differently than the M reeds for the white keys!
If you remove the grille and look inside the cassotto, the first row of reeds (under the pallets) is the M reed for the black keys, the second row is the L reed for the black keys, then the M reed for the white keys and finally the L reed for the white keys. That is, on a piano accordion.
Because the M reeds for the black keys are so close to the "exit" of the cassotto their sound comes out more directly, not benefitting as much from the chamber as the reeds that are deeper inside. So the M reeds of the black keys sound less mellow (which you identify as having a "slight buzz". As there are 7 white keys versus 5 black ones in every octave many accordions try to even this out by have one or more white keys mixed in with the black keys on that first reed block. If you play all the white keys in the M register you can identify which are on the wrong block as they are a bit less mellow. Don't try this on a Hohner Gola or Morino as they have a strict white/black separation so none of the white keys have the sound coming from the first reed block.
On a large button accordion there are three reed blocks in the cassotto. Looking into the cassotto the first reeds are M, then L for the 3rd row of buttons, then M and L for the 2nd row and deepest inside are M and L for the 1st row. You will notice that the M reeds for the third row sound less mellow than the other reeds.
The slightly less mellow M sound on black keys has prompted many jazz players to play in just the L register, and this then prompted the construction of special jazz keyboard that starts at low G and goes up to high C, to give you more high notes that are needed when using just the L register.
