First, a BIG congrats on a 1926 dated M91 (Tula Arsenal, I suspect). These are rarely encountered, as the M91 was being phased out through the 1920's, and very few are recorded to be from the final year of production.

Next, and again I'll speak in general terms, any bolt in any Finn rifle has a chance to be swapped from another rifle, unless matched to the original font on the barrel. My first impression is that most occurrences of blued bolts are found in Model M91/30's, and I don't recall ever seeing an example in an M91. Could it have been blued by the Nazis? It's possible, but it would be very difficult to guarantee.