When Microsoft first released the Windows 10 Mobile upgrade for older devices, Blu was one of only two third party OEMs with devices on the list. The only other third party device was the MCJ Madosma Q501, and the third OEM was just Nokia/Microsoft.
It would have appeared at the time that Blu was committed to Windows phones. After all, other companies had promised to upgrade their devices - such as LG, HTC, and Yezz - so Blu seemed to be part of some exclusive club (the Yezz Billy 4.7 still can't get an official upgrade, but it is now eligible for the Insider Program). To this day, we have no idea why some phones that should be considered Windows 10 Mobile hardware were omitted from official support.
It would appear that Blu isn't as serious about Windows phones as it once was, as it has removed mention of them from its homepage. Under Devices, the options are now Android and Feature.
Blu has long been a maker of Windows phones with its Win Jr and Win HD series, which was followed by a second generation "LTE" model of each. It would appear that a third generation isn't to be expected.
We've reached out to Blu to see if there's anything that they'd like to share regarding this matter, specifically whether the company will continue to make new Windows phones, and if it will continue to support its existing lineup.