In the 8th bar of this piece, when the 6 4 time signature starts, there is a minim/half note, with a black stem. How does one interpret the length of this note? Also, the length of the other notes with two stems?

Wow, this is interesting!

The image you provided is from the original Paris print from Maurice Schlesinger. This notation is also found in a Breitkopf und Härtel version from 1878; an 1881 version co-issued by Schirmer and a lesser-known Berlin company; another Schirmer print from 1894 edited by Mikuli; and a 1915 print from Schirmer, now edited by Rasael Joseffy. (I think I got all of those links correct; of course my Internet would get fussy as I was trying to do that!)

A Leipzig print from C. F. Peters in 1879 just switches the stems around, but the first three pitches of this group still seem to have one eighth-note too few to last until the end of the measure:

Karl Klindworth's edition (Berlin, 1880) finally gets it right! (Karl Klindworth is perhaps most famous for creating the piano reductions of Wagner's Ring cycle...no small task!)

Notice a few things here:

• The opening C4 is now tied to another C4 half-note, which now will last to the end of the bar.
• The dot is moved from the D to the F#! This means that now:
• The D, as a half-note, will also last to the end of the bar, as will:
• The F#, whose new status as a dotted-quarter also lasts to the end of the bar.

In short, just hold every damn note to the end except the B-flat :-) This makes sense; Chopin is just asking you to sustain the pitches of the D7 chord which will resolve to G minor in the next measure. Furthermore, the specified fingering of 5-5 on the B-flat to A makes it impossible to hold that B-flat to the end. (Note also the pedal marking underneath!)

Now, I said above that Klindworth finally got it "right." The only real way to determine whether or not this is right is to check out Chopin's own original manuscript. (Though it's certainly possible that he made a mistake, as well!) But Klindworth's score certainly makes much more sense than the previous versions.

It seems that the original print made an error, and several future prints followed this error. It wasn't until Klindworth came along that he seemed to notice the issue and fix it.

• The notation is not an issue - the "fix" is, in fact, incorrect. Look at where the pedal is lifted: the notes aren't held underneath that last quaver; the last quaver is dry.
– user16935
Jul 20, 2016 at 13:34
• I don't understand what you mean. The notation is absolutely an issue! And yes, although the pedal may lift, the notes are held through. The lifting of the pedal just prevents the muddling up of the B-flat to A. Jul 20, 2016 at 13:37
• No. The lifting of the pedal allows the B♭ to A to transition from accompaniment to melody notes. The A leads into G in the melody in m.9. He repeats this ambiguity throughout the theme. Dropping the underlying notes allows the texture to thin out while this is happening. The original notation is idiosyncratic, but it is not wrong.
– user16935
Jul 20, 2016 at 14:25