Okay so say I have champion of lambholt out with umbral mantle equipped. Attack phase, I swing with champ. my opponent declares blockers (say a 1/1 token) but before combat damage is dealt I untap with umbral mantle making champion a 3/3. Would the champions ability take effect or would she be blocked? Similarly what if I did this before blockers were declared?
In this case, Champion of Lambholt would still be blocked. Changing its power after blockers are declared won't undo the block or remove creatures from combat (C.R. 506.4a). Abilities that prevent creatures from blocking are relevant only at the moment blockers are declared as the declare blockers step begins (C.R. 509.1b-c), and not at any other time.
If you activated the ability granted by Umbral Mantle before blockers are declared, creatures with power less than Champion of Lambholt's current power (which, without more, will be 3) won't be able to block Champion of Lambholt.