I've often heard it claimed blade grinders cannot grind fine enough to make espresso, but is this true? I've run my cheap blade grinder continuously until its humming sound becomes steady and smooth (about 30 seconds), indicating the blade isn't hitting any larger-sized chunks, and it produces very fine, seemingly homogeneous, powdery grinds, and works well with my moka.
Blade grinders cannot produce homogenous grinds, even if they may seem so to the eye. Additionally, you can't control grind size precisely and consistently, which is important for espresso.
Grind size and size distribution, have a crucial impact on extraction under the conditions of an espresso machine (high pressure, short extraction time). An important part of "dialing in" a good espresso shot consists of finding the correct grind, which then needs to be reproduced consistently for subsequent shots.
Moka pots are much more forgiving, due to their lower pressure and longer extraction time, and do best with a considerably coarser grind than espresso machines.
Krupps F203 blade grinder manual says (p. 7):
Grinding for Espresso/Cappuccino
The Krups Fast-touch (F203) is acceptable for grinding beans for boiler type espresso machines. We do not recommend using the Fast-Touch to grind beans for use with pump espresso machines. Pump espresso machines require a disc type [burr] grinder such as the KRUPS «II Barista» ref 220 or a similar, professional style espresso grinder. This type of grinder will prevent coffee dust which can impede the flow of water through the espresso machine.
They seem to think coffee dust wouldn't be a problem for "boiler type espresso machines" (e.g., mokas).
Thus, blade grinders can be used for mokas, though "coffee dust" may come through.
U̲p̲d̲a̲t̲e̲: I bought a Krups GVX2-12 and noticed it grinds much more homogeneous-sized grains and nothing finer ("coffee dust"); consequently, there aren't over-extraction, bitter issues due to the "coffee dust" of the blade grinder.