A dark blue colour.According to the quote, they called them "spherules", not "blueberries"
A mineral formation first identified by the Mars Exploration Rover Opportunity in 2004, so-named because of its resemblance to blueberry muffins. We see these strange round objects we're calling “spherules” embedded in the outcrop, like blueberries in a muffin.
1939, Kathrene Pinkerton, Wilderness Life, Carrick and Evans (1939), page 179:We blueberried on an open flat beside the river. The ground was covered with great frosted blue globules, sweet and warm in the sunshine.
1947, Robert Wallcott & Albert Hale, "What People Talk About", Daily Boston Globe, 26 August 1947:The "white longlegged, long-necked bird" seen by your Ayer reader while she was blueberrying on the shore of a pond was either the Little Blue Heron in white phase or immature,
1951, Elizabeth Coatsworth, The Enchanted: An Incredible Tale, Pantheon (1951), page 62:They had not passed again in the surrey going to the Forks, nine miles away, and none of the girls had been blueberrying among the bushes at the edge of the woods.
1988, Ms. Magazine, Volume 17, Issues 1-6, page 38:Sarah and I have been blueberrying together off and on since the summer of '64. This morning, armed with our pots and pans, we went out and picked two quarts of wild berries and then came home and made a cake.
2000, Robert Dash, Notes from Madoo: Making a Garden in the Hamptons, Houghton Mifflin Company (2000), ISBN 9780618016921, page 152:Pointy fraise de bois went through it all with undiminished generosity (so small a plant for all that giving!) and the picking was fine, for the birds were off blueberrying and taking the late raspberries just as they ripened.
2000, Edward Hoagland, "A Peaceable Kingdom", in Tigers & Ice: Reflections on Nature and Life, The Lyons Press (2000), ISBN 9781585741823, page 61:On some of the richest days, when a moose stalks by or a bear is blueberrying or munching hazelnuts outside, I think of my house as a bathysphere suspended in the wilderness.
2002, Loretta Ellsworth, The Shrouding Woman, Henry Holt and Company (2002), ISBN 9781429932462, unnumbered page:"Come, Aunt Flo. I'll show you where we go blueberrying. Last year we got almost a bushel of berries, and Papa says they should be ripe now."
2002, Lois Kenyon Pesanelli, His Hand Upon Me for Miracles, 1st Books Library (2002), ISBN 140330095X, page 14:We decided to go blueberrying one day up in our hills. We grabbed our blueberry cans, hitched them to our belts, and headed for the blueberries.
