Copper II oxide is a base but not an alkali. An alkali is a soluble base. Since Copper II oxide is not soluble in water then it is not an alkali.
Let us recall that the change of colour of litmus with an alkali requires the presence of water. In the absence of water, solid Copper II oxide does not turn red litmus paper blue.
The ability to turn red litmus paper blue is commonly observed with alkalis and Copper II oxide is not an alkali.
Also recall that since Copper II oxide is not soluble, hydroxide ions are absent hence Copper II oxide does not turn red litmus paper blue.
