Using just statistics, here is where each phone excels the other in:
The Optimus L9 has a 4.5 inch screen whereas the Blaze has a 4-inch screen.
The Optimus has a resolution of 540x960 and the Blaze has a resolution of a 480x800.
The Optimus L9 has a ppi (pixels per inch) of 246 whereas the Blaze has a ppi of 235.
However, the Blaze has a Super AMOLED screen which is, reported by me and other Super AMOLED screen users, looks much better than the Optimus L9's LCD screen.
The Blaze has a faster 1.5Ghz Qualcomm Snapdragon S3 APQ8060 dual-core processor compared to the L9's 1.0Ghz dual-core TI OMAP4430.
The L9 has a battery of 2150mAh and the Blaze has a battery of 1750mAh. Note that the battery life of the L9 might not be significantly better than the Blaze with the screen on since the L9's screen is larger which consumes more battery.
Both have 5 MP rear cameras. However, the L9 shoots videos in 1080p whereas the Blaze only shoots in 720p.
The front camera for the Blaze is 1.3MP and the L9's is 0.3MP.
Theoretically, the Blaze should get faster internet speeds with it's HSPA+ radio whereas the L9 has an unspecified HSPA radio.
Both come with 4GB of internal storage with the ability to use up to a 32GB microSD card.