There are a number of different birds in the sparrow family. In Massachusetts, USA, I think what we technically have most of is the House Sparrow. This page has a whole sparrow list with their sounds. Your profile indicates that you live in California. There are lists here and here of the most common sparrows and small birds in California. Some California sparrow and related species are threatened or endangered.
We have at least 30 sparrow-looking birds on any given winter day, and many more in the summer. We also have many other birds, like blue jays and cardinals, and lots of squirrels. For that reason, we have many different shapes of feeders!
Why aren't they eating out of the clean bowl?
Our experience exactly matches what many experts say, that house sparrows and other birds in that family are mostly ground feeders. They enjoy bushes too, where they can hide while they eat. Infrequently they use a hanging feeder, but they don't like it when the feeder moves in the wind. In their natural habitat, the food they like, including grains and insects, are pretty much strewn around, or close to, the ground. We also have a lot of luck throwing seeds in the bushes, which is also part of their natural eating and nesting habitat.
Although many can be drawn to feeders, especially open platform feeders, most sparrows prefer eating on the ground, especially near trees, shrubs, or a brush pile, where they can make a quick getaway. Feeders might attract half a dozen sparrows in a yard where a good ground-feeding area would attract dozens. Source.
Sparrows don't have the body or head shape to comfortably pick through a pile of seeds, so when given an opportunity, they'll likely choose the seeds they grab while hopping. I also don't know about the depth or type of edges of the bowl you're using, but they may not feel steady and secure. I haven't tried feeding them from a bowl, so I don't want to give what could be wrong advice.
Is the blue color scaring them away?
My initial reaction was that color was likely irrelevant. However, some studies show clear differences among these birds, and blue is a preferred color among sparrows, so I doubt that's the problem in this case. In fact, if you put out a blue tray or other type of feeder, you may even see more!
The most in-depth studies I found were conducted in England. Results showed that given the choice of different colors, the sparrows' two favorites are green and blue, especially in summer. In the winter, some birds preferred silver, most likely because of the way they perceive color, which is different and more sophisticated than humans.
Colour preferences of UK garden birds at supplementary seed feeders, published in 2017 by the U.S. National Institute of Health, is long and technical, but really fascinating.
The primary aim of our research was to investigate the effect of feeder colour on the feeding preferences of wild birds.
To explore the effect of colour on the number of visits by birds, we recorded bird visit rates to 8 different coloured feeders at three sites on 78 sampling days during the winter/spring of 2014/15 (November 2014 to May 2015).
The colors were red, yellow, green, blue, purple, white, silver and black. They monitored several species of backyard visitors, including sparrows. Their research showed that sparrows preferred green, with blue a very close second. Here is a detailed graph, separated by birds and by their chosen color.
The highly respected Royal Society for the Protection of Birds reported on a similar study by the British Trust for Ornithology in Do different colored bird feeders attract different birds?. They used more colors and a broader group of birds. Their findings varied among the species, and the types of food they ate. Nevertheless, it's interesting to note that many of the backyard visitors preferred blue feeders in the summer, and silver in the winter.
In trials carried out by the British Trust for Ornithology, 12 different coloured feeders were used with a variety of food types to investigate feeder colour preferences in birds. Their popularity was measured and recorded by the amount of food consumed from each feeder. Further studies showed that for seed, blue feeders proved the most popular during the summer and that silver coloured feeders were popular all year round.