A blue plastic bag filters most of the spectrum EXCEPT the blue part. Not the other way around. Simple proof: look inside, the light is blue.
I guess the biggest problem for the plant here is that especially the infrared part of the sunlight is very important. Yellow leaves usually mean not enough light. Especially tomatoes need lots of sunlight.
No, a blue filter, like a blue plastic bag would let blue pass and absorbe the other colors. That's why if you shine a white light through a blue filter, things look blue on the other side.
Now, some of the blue light is scattered away, and that's why you see the plastic bag itself as blue.
Hang on, so if you shine a white light through a blue filter, the light on the other side looks blue. And the filter itself still looks blue on both sides. But then, where did the non-blue component of the light go?
I guess the biggest problem for the plant here is that especially the infrared part of the sunlight is very important. Yellow leaves usually mean not enough light. Especially tomatoes need lots of sunlight.