A company has started selling the first blood test to help diagnose Alzheimer’s disease. It’s a long-sought goal that could make it much easier for people to learn whether they have this most common form of dementia. But independent experts are leery. Results to support claims about the test’s accuracy have not been published nor has the test been approved by the Food and Drug Administration. The test costs more than $1,000 and is not covered by insurance. Brain scans that cost roughly $5,000 are the gold standard for helping to diagnose the disease now.