Hi Nothinghead,
Based on your VEHICLE'S V.I.N. :
C=Chevrolet
K=4x4
U=305 V8
1=half ton
8=Blazer
9=1979
F=Flint, MICHIGAN
190942=Build #
The 305 motor that WAS in your Blazer would have one of the following suffixes: TYR TYS UTA UTB UYR or UYS
The 350 motor that IS in your Blazer could be from a vehicle that matches the codes you observed, I'm not very good with Engine Codes. I DO know that the "TKY" block designation was also in 1980 C/K Blazers also.
IMHO, the motor is NOT original to the vehicle. Doesn't sound like its bad news though, btw....