This is a very good question.
I suspect the reason that you have had no replies is that no one knows what a blast air line is.
Apart from the blast air lines (Tuyeres) in a steel blast furnace, the only blast air lines I know of are completely obsolete.
In the early days of the dastardly Dr Diesel's invention, a measured quantity of fuel (initially powdered coal) was injected into the cylinders using high pressure compressed air, this was known as "Blast Injection".
After problems using coal, the same system was modified to use liquid fuel. (Diesel oil).
The air compressors were engine driven, and took about 40% of the power of the engine to drive them!
Engine design rapidly changed to using "solid injection" which is what we are all used to now.
Unless anyone knows of any other blast air lines, the question is about 80 years out of date!!
The reasons for an explosion in this line would be similar to that for an air start line.
1) Fuel leaking past the metering device into the air line, and being heated and compressed when the blast air is admitted to the line, causing "diesel ignition".
2) The valve between the the cylinder and the blast air system leaking and allowing the cylinder temperatures and pressures into the fuel metering device and blast air line.
Good luck with the exams.
It is always better to ask a stupid question than to do a stupid thing.