The addition to concrete of calcium chloride or admixtures containing calcium chloride can darken a concrete surface. Light spot or dark spot (mottled) discoloration can occur depending on the curing method and the alkali content of the cement.
Kosmatka, Steven H. "Discoloration of Concrete - Causes and Remedies" Portland Cement Association Document PL861

Although the addition of calcium chloride accelerates setting and hardening of cement, it does not necessarily follow that when added to individual components of cement, it acts as an accelerator. For example, calcium chloride retards the hydration of the tricalcium aluminate phase, while acting as an accelerator for the hydration of the silicate phase.

In the presence of larger amounts of CaCl2, more Ca ions enter the C-S-H, forming a product with a larger than normal C/S ratio.

...calcium chloride may react with the aluminate and ferrite phases and it may exist in different states in the tricalcium silicate-calcium chloride-water system.
A dosage expressed in terms of a certain percentage of calcium chloride will be ambiguous because it might refer to the pure anhydrous calcium chloride or to the hydrate... According to the standards, the regular flake form should contain a minimum of 77% calcium chloride (anhydrous) and the pellet or other granular forms, a minimum of 94% calcium chloride. In these two types, the percentage of calcium chloride may vary depending on the purity. The use of specific language would ensure the use of the right amount. Dosage should be expressed in terms of "percent calcium chloride dihydrate" or "percent anhydrous calcium chloride" or by the chemical formula CaCl2 or CaCl2*2H2O, or chloride ions.
Ramachandran, V.S. "Concrete Admixtures Handbook - Properties, Science and Technology" Noyes Publications, Park Ridge:New Jersey, pp. 247-249

Primary factors contributing to discoloration of flatwork.
In these studies, no single factor seemed to cause discoloration. However, combinations of factors caused very severe discoloration. Factors found to influence discoloration were calcium chloride admixtures, cement alkalies, hard troweled surfaces, inadequate or inappropriate curing, concreting practices and finishing procedures that cause surface variation of water-cement ratio, and changes in the concrete mix…(p. 39)
Conclusions:
Calcium chloride in concrete is a primary cause of concrete discoloration. The chances for discoloration are much less if calcium chloride is not used.
Discoloration becomes more pronounced and more permanent with harder troweling. (from Conclusions, p. 48)
Greening and Landgren. "Surface Discoloration of Concrete Flatwork." Journal of the Portland Cement Association, Vol 8, No 3, pp 34-50 September, 1966 (Italics added)