There is a huge range of effectiveness with red blinking taillights. From almost useless to daylight visible. In most cases attaching an LED blinker to a bag or yourself is almost useless. For an LED blinker to work it needs to be aimed horizontal and to stay that way all the time. The bag mount will move around too much. The new lights have better LED's, they also have lenses to multiply the effect, and they address side visibility properly with lights and lenses facing the side that are visible from the side and even well forward.
Put reflectors on the bag if you like and all sorts of reflective material on anything.
If you don't have a new style LED taillight, get the new Cateye TL-LD 1000, bolt it to you rack or your seat post, make sure it is aimed straight back, and go for a ride in the dark. If you are not familiar with the new lights you will be surprised.