This is called bronzing. It's the result of the way pigment inks work, and is only evident in heavily-shaded areas because of the use of pigment black ink alongside dye color inks. Unfortunately, the issue is inherent to the design of this and many other consumer inkjet printers. It may be possible to work around the issue by using only the tricolor cartridge and having the machine produce a composite black by mixing the three dye color inks, but I don't know if the Tango X or the Instant Ink service allows this.

Read on for a technical explanation of how pigment inks can cause bronzing...

Dye inks are absorbed into the paper; pigments adhere to the surface of the page. Pigment ink is used for black because it's water-resistant and produces sharper output, which is ideal for text and line graphics. Dyes are used for colors as it's easier to produce a wider color gamut with dyes than pigments. Canon has a good explanation of the differences between dye and pigment inks in this article.

The use of pigment black with dye colors is so that the printer can produce both sharp text and vivid colors in graphics. But because pigment ink isn't actually absorbed into the paper, you can end up with a shimmering or iridescent effect when used on photo paper. This is what we call bronzing. For this reason, less expensive consumer photo printers may use only dye inks. (Paradoxically, professional photo printers often use all pigment inks for print longevity. While they use more advanced inks that are less prone to bronzing, they're generally much more expensive.)

Many Canon consumer inkjet printers use a five-ink Hybrid Ink System with both pigment and dye black inks to produce more vivid colors and get around this problem when printing photos. On smaller models that use a combined color cartridge, the dye black is included in the color cartridge and the black cartridge contains only pigment ink. On larger machines with individual cartridges for each color, dye black is a fifth cartridge separate from the pigment black and dye colors.

A few older HP printers use a Photo cartridge with dye black ink, in addition to the usual pigment black and dye tricolor cartridges, but AFAICT they no longer make printers with this setup.