What happens to foreign chemicals/particles in the blood which cannot be metabolised by the liver? are they still excreted somehow or will they remain in the blood?

Nearly all chemicals are excreted by the kidney or metabolized by the liver and then excreted through the bile. Larger particles which may not be degradable are captured/engulfed by the Reticuloendothelial system(RES) of the liver and the spleen and slowly degraded.

It depends on the chemical. Lead and uranium and other heavy metals get stored in bones. Other chemicals like dioxin and DDT are stored in fat. Volatile chemicals could get removed through the lungs with expired air.