I purchased a ginseng ficus bonsai from IKEA about 3 weeks ago and half of its leaves have fallen off and/or are turning black. The room gets low/medium light with windows facing west and north. Please see the attached picture. This isn’t my first ficus from ikea and the previous one has been doing swell for over 2 years now. Any thoughts?

Look, nothing against IKEA, but many problems here:
1-) It's not a bonsai.
2-) The pot+tray are not bonsai traditional pots
3-) The soil doesn't look very good
4-) There is reverse taper on the trunk
5-) It needs more light
6-) it needs more air circulation

If there isn't enough light and air circulation than it will always have too much water.

Jrjsweeney, in order to try helping you with this problem I´m posting an image with the main/most usual problems that might occur to a tree´s leaves. The picture itself is written in Spanish, but I´m posting its translation here for you to understand it. Oh, the first 2 cases correspond to new leaves growth and the next 6 examples correspond to older or established leaves. (That is what the black line separates)

Hope this helps,

#1 Severe Nitrogen deficiency.
Small new leaves presenting a white/yellow coloration.

#2 Calcium deficiency
New pale and wrinkled growths (It could also be an overdose of potassium, magnesium)

#3 Iron Deficiency
Greenish nerves and old yellowish leaves

#4 Phosphate Deficiency (Phosphorus)
Older leaves presenting a yellow color with dark spots. The leaves die and fall rapidly (Note the similarity to early nitrogen deficiency)

#5 Magnesium Deficiency
Dark veins and old leaves with an extreme light green tint or shade

#6 Normal Leaf

#7 Early Signs of Nitrogen Deficiency.
Older leaves turn yellow, die and fall

#8 Potassium Deficiency
Very small holes with yellowish edges.