Jennifer,
There is nothing that can be tested by a "simple" blood test that will be able to tell you if your child has CF or not. There are many things that can cause poor weight gain, and they are wise to look into some or all of them.
Genetic testing is the only blood test that will tell you if she has CF or not. Most of the time doctors will order a sweat test before genetic testing is considered. A sweat test is way less invasive (not painful like obtaining blood can be) and fairly inexpensive. It is a very reasonable place to start. There is no reason to not do the sweat test if your daughter is not gaining weight.
There is no one way that CFer's look. Read this board and you will see how different the range of symptoms are. There are many people who were first tested for CF because of poor weight gain.
I hope that you find there is some simple issue with your daughter that you are able to easily fix. If you have any thoughts that she may have CF push to have her tested. Can you take her to a different doctor? Even if you tried a GI doctor, they can order the test for you.
BTW- I ditto what Mel said about numbers!
Good luck and keep us posted!