Well sure, it would work. But you'd still have to pick some resistors that suited your blue LEDs. And it seems a complex way to go about swapping some LEDs!!
I still say simply swapping in some blue LEDs will be fine - as long as you find some that are low brightness. Most of the previously suggested LEDs have been ultra-bright types, 2000mcd or more, and they would need a whopping resistor (maybe 10k or more) to get them to be dim enough.
Alternatively, pick some bright ones and fit some extra resistors somewhere. Even that is simpler than building my converter just for LEDs!
The big problem is not knowing what resistor value will work nicely with any given LED. I usually find it's a case of trial and error, when running them at a low level. So unless you want to stock up on a wide selection of resistors, it's best to guess and live with the consequences
We have to make a guess anyway as to what the original LED's mcd rating is - I'd say about 10mcd. So using the same reisistors, you'd probably want blue LEDs with less than 4V Vf and about 15 to 20mcd. Perhaps
these... it looks like there's quite a few to choose from.
edit: all of this is assuming that there's nothing too unusual in how the LEDs are powered. There's a question mark over that until we know what voltage (probably 0V) that the chips put onto the resistors.
